Explanation:

Time = 100(x – 1) / r
Where x is the no. of times the sum becomes of itself.
Here the sum is getting 2 times. Therefore, x = 2
Where r is the rate of interest = 25/4%
By the short trick approach, we get
= 100(2 – 1) / 25/4
= 100 x 4/25 = 16 yrs