Julia played tag with 23 kids on monday, 15 kids on tuesday and 23 kids on wednesday. How many more kids did she play with on monday than on tuesday?
Julia played with 23 kids on Monday. 
On Tuesday, she played with 15 kids. 
That means she played with 8 fewer kids on Tuesday than on Monday. 
On Wednesday, she played with 23 kids again. 
That means she played with the same number of kids on Wednesday as she did on Monday. 

So the answer is 8.