Median means "middle". When finding the median, think of finding the middle number(s). In set A, there is an even amount of numbers, so there is no exact middle number, but there are two numbers in the middle- which are 0 and -2. You get to zero and two by counting how many numbers there are, and if it is even, pick the two numbers in the middle. If there is an odd amount, you pick the one number in the middle- like in the next problem. Simply count to the number in the middle by making sure there is an even amount of numbers on bith sides, which will lead you to a median of 1.5. See how there's 2 numbers on both sides of 1.5? That lets you know that you found the correct median because both sides have the same amount of numbers left over, not an uneven amount.
