"How much is twenty plus thirty?"
Translation:Combien font vingt plus trente ?
It is my understanding that "combien font" is the most commonly used phrase when asking "how much is" questions when dealing with multiple numbers.
Also, remember that égaler, like faire, is a verb so would be conjugated in the plural - hence "égalent ".
Perhaps "Combien égalent vingt plus trente ? " is accepted? I can't say.
For some interested reading see these links:
Read the previous posts, especially the one by Sitesurf above yours. The use of faire here is idiomatic.
In a similar vein, when you talk about the total of measurements, you use faire.
Cet appartement fait 50 m2. "This apartment is 50 square metres."
L’étagère fait 100cm de long, 120cm de large, et 40cm de profondeur.
"The shelf is 100cm long, 120cm wide, and 40 cm deep."