https://www.duolingo.com/SussexSoleil

A maths question in French for intermediate learners to practice.

Sophia décide de faire une course à bicyclette avec son amie Linda. Comme Sophia est plus rapide, elle laisse à son amie 30 minutes d’avance. Si Sophia roule à une vitesse moyenne de 30 km/h et que Linda va à 25 km/h, après combien de temps se rejoindront-elles et quelle distance leur restera–t-il à parcourir, si la course est de 90 km?

July 28, 2015

29 Comments


https://www.duolingo.com/panagiotis_ts

Malheureusement je suis NUL NUL NUL en mathématiques à l'école, alors je ne peux pas calculer tout ça même en ma langue maternelle. :/ Mais résoudre des problèmes de maths en une langue étrangère peut être très utile pour pratiquer cette langue (au moins pour ceux/celles qui ont des connaissances suffisantes en maths). :-)

July 28, 2015

https://www.duolingo.com/SussexSoleil

C'est dommage, mais merci quand même !

July 28, 2015

https://www.duolingo.com/DonFiore

Someone else can explain it in French, but:

  • Linda gets a head start of 25 x 0,5 = 12,5 km
  • Sophia rides 30 - 25 = 5 km/h faster so
  • she catches Linda in 12,5 / 5 = 2,5 hours.
  • 30 x 2,5 = 25 x 3 = 75 km.
  • They have 90 - 75 = 15 km left.
July 28, 2015

https://www.duolingo.com/Bvogel1

Where were you when I was in the fifth grade?? I never understood this stuff!

July 28, 2015

https://www.duolingo.com/panagiotis_ts

Enfin! Quelqu'un qui peut me comprendre! En septembre je serai en seconde (au lycée) et je ne peux pas résoudre un tel problème en maths! :o (bien que je sois fort en langues :D)

July 28, 2015

https://www.duolingo.com/SussexSoleil

Bien fait!

July 28, 2015

https://www.duolingo.com/ketoacidosis

Fantastique!

Linda avance (25 km/h) * (0,5 h) = 12,5 km après son amie commence. Cette distance diminue 5 km par heure. Elles sont ensemble dans (12,5 km) / (5 km/h) = 2,5 heures plus tard (depuis Sophia a commencé). La distance que reste est (90 km) - (30 km/h)*(2,5 h) = 15 km.

En anglais (parce qu'il ya beaucoup de erreurs en français):

Linda advances 12.5 km before her friend start. That distance decreases by 5 km per hour. They are together 2.5 hours later from the time that Sophia started, or 3 hours from the time that Linda started. The remaining distance is 15 km.

July 29, 2015

https://www.duolingo.com/SussexSoleil

Chouette! Bien expliqué.

Another nice way of looking at the problem without solving algebraic equations for t.

July 29, 2015

https://www.duolingo.com/andy6262

Another way to think about the problem. Let t=0 denote the time that Sophia starts. Then at any positive value of t, Linda has traveled a distance of 25 x t+12.5 km (or 25 x t+25 x 0.5) and Sophia has traveled a distance of 30 x t km. We can then set these distances equal to each other and solve for t.

25 x t + 12.5 = 30 x t

t=2.5 hr

It is the same result as DonFiore gave, just a little different way to think about the problem.

July 28, 2015

https://www.duolingo.com/SussexSoleil

Certainly. Although I found it slightly easier to set t = 0 hours when Linda starts.

Linda covers x km in time t hours, where

x = t * 25 km/h

Sophia covers y km in time (t - 0.5) hours, where

y = (t - 0.5) * 30 km/h

When they meet, x = y, t = 3, x = 3 * 25 km/h = 75 km, etc.

July 28, 2015

https://www.duolingo.com/Bvogel1

Sorry... I have a better chance of getting to Level 25 in every language offered on Duolingo than I do of figuring out a word problem. But have a lingot for creativity.

July 28, 2015

https://www.duolingo.com/Tamuna10

Comme moi et mon frere avons fait, elles vont se rejoindre en 2.5 heures, et il les restera 15 km à parcourir :) Avons nous raison?

July 28, 2015

https://www.duolingo.com/SussexSoleil

Malheur! Vous avez fait une erreur de distance.

En 3,0 heures, à 25 km/h, Linda a fait combien de kilometres ? Elle lui reste combien a faire ?

July 28, 2015

https://www.duolingo.com/Tamuna10

Oh ma faute, il restera 15 km a faire xD excuse-moi. Je modifierai. J'ai ecrit 15 km d'abord et ici j'ai ecrit 25.. :))

July 28, 2015

https://www.duolingo.com/SussexSoleil

I've just realised after working it out for myself, that the correct answer to the question:

après combien de temps se rejoindront-elles ?

is not 2.5 heures !

Please fix your answers, or I'll have to claim my Lingots back :-)

July 28, 2015

https://www.duolingo.com/andy6262

The question is a bit vague since it could interpreted as the amount of time after Sophia starts of the amount of time after Linda starts.

July 28, 2015

https://www.duolingo.com/SussexSoleil

As this is a question of French comprehension, I think I need to be a little bit strict here !

Fr. "se rejoindre" means that two or more parties meet up again. The question doesn't say how long does it take Sophia to catch up with Linda.

July 28, 2015

https://www.duolingo.com/andy6262

Right, but it is ambiguous as to when we start the clock. If Linda left thirty minutes ago and Sophia leaves right now, they will meet up again 2.5 hrs from now. If Linda leaves right now, and Sophia will stand around for the next thirty minutes, they will meet up again in 3 hrs.

The solution is at http://archimede.mat.ulaval.ca/amq/bulletins/dec08/Chronique_enigmes%28dec08%29.pdf. Both values are given, the amount of time after Linda starts and the amount of time after Sophia starts. Notice it says "Sophia rejoindra Linda apres 2,5 heures, donc 3 heures apres le depart de Linda."

July 28, 2015

https://www.duolingo.com/SussexSoleil

I'm very tempted to say, mathematically, that I agree with you! However, as we are supposed to be learning French, and at the risk of seeming very boorish and pedantic ... :

It is important to recognise that the verb "rejoindre". as used in the solution linked to, is transitive. This means there is a subject A performing the action "rejoindre" on object B. So it is OK to use it to mean Sophia (A) "catches up with" Linda (B) after 2.5 hours.

However, "se rejoindre" is a different kettle of fish. The "se" prefix turns it into a pronominal, reciprocal verb. Reciprocal verbs require two or more subjects (Sophia and Linda) and these parties are also the objects of the action in equal measure. As it is not possible for Sophia to meet Linda after 2.5 hours, but Linda to meet Sophia 3 hours after their last meeting, the only grammatically logical answer is to include Sophia's waiting time in the time they have been apart.

July 28, 2015

https://www.duolingo.com/andy6262

I think we are just going to have to agree to disagree on this one. Yes, they can meet up three hours after their previous meeting, but my opinion they can also meet up two and a half hours after Sophia starts riding.

July 28, 2015

https://www.duolingo.com/SussexSoleil

Oh, go on then. It would be rude of me to argue further with a guest post. And I was only joking about wanting my Lingots back. Have another.

July 29, 2015

https://www.duolingo.com/Tamuna10

Then what's the right answer? :/

July 28, 2015

https://www.duolingo.com/SussexSoleil

It is 3 hours.

Say the race starts at 09:00. Linda sets off, leaving Sophia behind. Sophia leaves at 09:30, having given Linda a 30 minute head start. Sophia finally catches up with Linda at 12:00. They have been apart for 3 hours.

July 28, 2015

https://www.duolingo.com/Tamuna10

But if course is 90 km long and Sophia runs with 30 km/h, she will finish the race in 3 hours :/ Wouldn't have she caught up with Linda earlier than finishing the race? :/ Sophia will need 2,5 hours to catch up with Linda, and they will meet each other at 75 km.

July 28, 2015

https://www.duolingo.com/SussexSoleil

Good argument, but you are forgetting that it takes Sophia 3 1/2 hours to finish the race; 30 minutes waiting time, plus 3 hours riding time. So after only 3 hours, she does catch up with Linda before she finishes the race.

Or look at it the other way around. If they start the race together, Sophia reaches the finish line in 3 hours, but Linda takes (90 km / 25 km/h) = 3.6 hours. So over the 90 km course, they have been apart for 3.6 hours.

If they instead stop the race at the 75 km point, Sophia gets there in less than 3 hours, but Linda still needs (75 km / 25 km/h) = 3 hours. They have been apart for 3 hours, whether Sophia waits at the beginning or the end of the race.

July 28, 2015
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