15 campers went rowing in the morning. Some more campers went rowing in the afternoon. If a total of 32 campers went rowing that day how many campers went rowing in the afternoon?
Let's say that x campers went rowing in the morning and y campers went rowing in the afternoon. 

Then we know that: 
x + y = 32
x = 15

We can solve this system of equations using substitution: 

y = 32 - 15
y = 17