When x is subtracted from each of 19, 28, 55 and 91. the numbers so obtained in this order are in proportion. What is the value of x?

7

The proportion : 

19 - x :  28 - x :: 55 - x : 91 - x 

Product of the middle terms = Product of the extreme terms 

(28 - x)(55 - x) = (19 - x)(91 - x) 

1540 - 83x + x²  =  1729 - 110x + x²

27x = 189

   x = 7