Question 9.

Let us assume the amount invested to be $P$, and the rate of interest to be $r$. 

Value of the investment after 3 years will be, $P\left(1+r\right)^3$

Value of the investment after 5 years will be, $P\left(1+r\right)^5$

$\left(1+r\right)^2=\frac{36}{25}$

$\left(1+r\right)^2=1.44$

$r=0.2$

We need to find the value of n for which $4000\left(1+r\right)^n \gt 20000$

$\left(1+r\right)^n \gt 5$

$\left(1.2\right)^n \gt 5$

We see that, 

$1.2^8=4.2999$

$1.2^9=5.15$

Hence it takes 9 years to grow to over 20,000.