Some campers went rowing in the morning while 9 campers went rowing in the afternoon. If a total of 4 campers went rowing that day How many campers went rowing in the morning?
There were 4 campers rowing in total. 
9 campers went rowing in the afternoon. 
That means that there were 9 + 4 = 13 campers rowing that day. 
But we know that there were only 13 campers rowing in total, so that means that there were 0 campers rowing in the morning.