how to solve it?

Question

if Chandran starts his investment with RM500 instead of RM300 at the same interest rate, calculate on which birthday will the total investment be more than RM25000 for the first time.

Answer 1

The same interest rate as what? We're not exactly looking at the same set of questions you are. Y'know?

What can we say based on this limited information?

25000 = 300r^t
250/3 = r^t
log(250/3) = t log(r)
t = log(250/3) / log(r)

25000 = 500r^T
.....
T = log(50) / log(r)

so T/t = log(50) / log(250/3)

we can compute the RATIO of the times, but not the times themselves, not without the interest rate.