When ‘ a’ is subtract from each of 21, 38, 55, and 106, then the numbers obtained in this order

When ‘ a’ is subtract from each of 21, 38, 55, and 106, then the numbers obtained in this order, are in proportion. What is the mean proportional between (8+a) and (a-1)?

a) 7

b) 6

c) 8

d) 4

 Sol: