w campers went rowing in the morning. Some more campers went rowing in the afternoon. If a total of x campers went rowing that day How many campers went rowing in the afternoon?
Let's say that w campers went rowing in the morning. 

Some more campers went rowing in the afternoon. 

Let's say that y campers went rowing in the afternoon. 

If a total of x campers went rowing that day, then: 

x = w + y 

y = x - w 

y = w + y - w 

y = w