A boy's parents gave him R30 000 for his 21st birthday. His parents said that they had invested a sum of money on his 10th birthday that had grown at 13,5% p.a. CI for 11 years. How much was invested 11 years ago?

The future value ($A$) is R30 000.
The annual interest rate ($i$) is 13.5% or 0.135.
The number of years ($n$) is 11.
We need to find the principal amount ($P$) that was invested.
The compound interest formula is:

$$A = P(1 + i)^n$$

$$30\,000 = P(1.135)^{11}$$ $$30\,000 = P(4.168716...)$$

$$P = \frac{30\,000}{4.168716...}$$ $$P \approx 7200.00$$

The amount invested 11 years ago was R7 200.00.

$$\text{R}7\,200.00$$