“How many minutes are in an hour?” Should also be accepted but currently isn’t.
It looks like a plural, but in this case it is not. It's oblique. When a noun is followed by a post-position (में) it changes to the oblique case. घंटा is a singular masculine noun ending in आ, so it's ending changes to ए. Other singular nouns don't change. Masculine plural nouns get an ओं ending when followed by a post-position, and feminine nouns get an इयों ending.
Thanks! I still don’t understand the concept of “oblique” case though... I’ve seen it before as part of some explanations, but this concept is foreign to me! Can someone find examples in either French or Spanish?!! Kinda lost here... I never know when to use which!
I can't help you with a French or Spanish example. But, here is a simple explanation in English. http://www.learning-hindi.com/post/1116750602/lesson-48-nouns-in-the-oblique-case
En français, c’est un cas (une modification du nom ou de l’adjectif) qui s’applique quand le nom est introduit par une préposition (c’est typiquement le cas d’un COI).
Le chat est dans la maison. La maison would be in the oblique case. Le chat is the subject of the verb, but la maison is the oblique. Yo me siento en la silla. Yo is the subject, en la silla is the oblique. Hope that's helpful.