There are x big monks and y little monks.

x+y= 100

3x+y/3= 100

X=25，y=75。

That is, 25 big monks and 75 little monks.

Method 2: Comparative method

If they were all big monks, they should have eaten 3* 100=300 fried dough sticks, but actually they only ate 100. There are 200 missing.

If the big monk eats 3- 1/3=8/3 more fried dough sticks than the young monk, then the number of young monks is:

200/(8/3)=600/8=75

The number of big monks is 100-75=25.