The Simple Interest after 10 years at some rate will be Rs.600 on a certain sum. If the principal is trebled after
5 years, what will be the total interest at the end of tenth year?

let the original rate be R percent per year, the principal amount be P, time T

Given is SI = 600 = $$\frac{P\times R\times T}{100}$$

Hence $$P\times R$$ = 6000

SI for first five years will be 300 at the end of first 5 years.

When P is trebled for next five years, the SI for those five years will be = $$\frac{3P\times R\times 5}{100}$$ = $$\frac{P\times R\times 15}{100}$$ =$$\frac{15\times 6000}{100}$$= 900

so, the total interest will be 300 + 900 = 1200