Not enough Information. I could say 50 and it would work. I could say 68 and it would work. Any even number thats under 80 but higher than what you think her eldest son is.
Answer:
It takes 1 year.
Step-by-step explanation:
You have to apply simple interest formula, I = (P×R×T)/100 where I represent interest amount, P is principle, R is interest rate and T is number of years :



