Well, I would just reverse it. It seems you just have to reverse the whole problem.
Median is the middle number.
Rearrange the numbers to smallest to biggest.
70, 77, 80, _, 82, 87, 88
If the median of the data set is 81 the middle number needs to be 81.
So the missing number is 81.
70, 77, 80, 81, 82, 87, 88
3/4 = or .75
Divide the numerator and denominator to reduce the fraction.
