What least number must be subtracted from each of the numbers 23, 30, 57 and 78, so that the remainders are in proportion?

If (x − 2), (x + 2), (2x + 1) and (2x + 19) are in proportion, find the value of x.

If (x + 5) is the geometric mean between (x + 2) and (x + 9), find the value of x.

Find two numbers whose mean proportion is 36 and the third proportional is 288.