What number must be subtracted from each of the numbers 55, 23, 43 and 19, so that the new values are in proportion?

7 

Let the required number = x,

The proportion =

(55 - x) : (23 - x) :: (43 - x) : (19 - x)

The product of middle terms = the product of extreme terms 

(23 - x)(43 - x) = (55 - x)(19 - x) 

989 - 66x + x² = 1045 - 74x + x²

                  8x = 56

                    x = 7

hence the required number is 7