100 mice eat 100 cakes. If each big mouse eats 3 cakes, and 3 baby mice eat 1 cake, how many big mice and baby mice are there?

1 big mouse = 3 cakes
3 baby mice = 1 cake
1 baby mouse = 1/3 cake
Suppose x = the amount of big mice
y=the amount of baby mice
100 mice —> x+y=100
100 cakes —> 3x+1/3 y=100
Then solve it like how you solve linear equation with 2 variables
3x+3y=300
3x+1/3 y=100
——————— -
8/3 y=200
y=75
x+75=100
x=25
Hence,there are total of 25 big mice and 75 baby mouse