Step-by-step explanation:
I have to assume that the interest rate is an annual rate, and therefore interest is calculated and added at the end of every year.
with r being the annual interest rate, y being the number of years, c is the beginning capital, we would have a formula
v(y) = c × (1 + r/100)^y
in our case
392000 = 112000 × (1 + r/100)¹⁴
3.5 = (1 + r/100)¹⁴
1.093608817... = 1 + r/100
0.093608817... = r/100
r = 9.3608817... % ≈ 9.36%
it must be invested at over 9.36%.
