My childhood friend hasn't read many books, but he's quite successful in business.

I've always been very curious. Today I visited him and finally found the answer!
His son was doing homework and couldn't solve a problem, so he asked me for help!
The question was: There are 15 chickens and rabbits in total, with 40 legs. How many chickens and rabbits are there?
I answered: "Let the number of chickens be X, and the number of rabbits be Y".....
I hadn't finished calculating the answer when my friend already gave it!
His method was: Assume chickens and rabbits are well-trained.
Blow a whistle, lift one foot, 40-15=25
Blow the whistle again, lift another foot, 25-15=10
At this point, all the chickens sit down, and the rabbits are still standing on two legs.
So, the rabbits are 10 ÷ 2 = 5, and the chickens are 15 - 5 = 10.
His son always scores first in math.
Such a method makes math teachers feel helpless!