Can someone help me with this problem please!?

Well I am doing box and whisker plots and i understand how to do them but the thing I dont understand is how to find a number in between two numbers for example 10 and 20 how do you find a number in between them? And it's not 15 it something you have to do with adding subtracting multiplying or dividing can you tell me the answer to the question then explain how you got that? Please, thanx!