Math, oh Math!

I was aiding in the classroom today and had stepped out for a minutes to help a little girl. As I walked back in, I saw these 3 boys (Aditya being one of them) furiously working on something.
Here is what had happened earlier:
Aditya  had no unfinished work while most of his classmates were ploughing through piles of it. So his teacher asked him if he wanted to try the next level of the problem of the month, he had already level A of the problem earlier. He was ready, along with a couple of second graders!

 As I went closer, I heard Aditya's table partner telling him how hard it was and how only a few second graders could do it, and Aditya is, in fact, a first grader. Aditya told him that he would still try and ask for help if he found it tough. The next thing I know, after about 5 minutes, Aditya was looking around the room, table cleared. As I approached him, he got up from his seat and whispered in my ear,"I am done with part B. But I will get the actual problem sheet only tomorrow. Then I will make a clear illustration and tell everybody that I found the answer. Till then don't tell anybody, please! Not even my teacher!"

Here is the Level B problem:
The Wheel Shop sells other kinds of vehicles. There are bicycles and go-carts in a different room of the shop. Each bicycle has only one seat and each go-cart has only one seat. There are a total of 21 seats and 54 wheels in that room.

How many are bicycles and how many are go-carts?
http://www.noycefdn.org/documents/math/POM/pom-wheelshop.pdf

I asked him if he had understood what the problem was and how come he found the answer so quick. "I just worked it out. It is fifteen and six."

I can bet that I would have taken longer to do this, even at the end of elementary school!