If a sum of money placed at compound interest doubles itself in 2.5 years. In how many years will it become eight times the original sum at the same rate?

Correct option is C

Given:
A sum becomes 2 times in 2.5 years.
Formula Used:
If a sum becomes x times in T years at compound interest, it becomes xⁿ times in n × T years.
Solution:
Here, the sum becomes 2 times in 2.5 years.
We need to find the time for it to become 8 times.
Since 8 can be written as 2³, we have n = 3.
Required time = 3 × 2.5 = 7.5 years.
Final Answer
So the correct answer is (c)