Tumi saves R800 per month for two years. She then invests this amount for 36 months at a simple interest rate of 12.5% p.a. Calculate the total interest earned.

The investment is for 36 months at 12.5% p.a. simple interest.
First, convert the time into years:

$$\text{Time } (t) = \frac{36 \text{ months}}{12 \text{ months/year}} = 3 \text{ years}$$

Convert the annual interest rate to a decimal:

$$\text{Interest rate } (r) = 12.5\% = \frac{12.5}{100} = 0.125$$

Use the simple interest formula $I = P \cdot r \cdot t$:

$$I = \text{R}19\,200 \times 0.125 \times 3$$ $$I = \text{R}2\,400 \times 3$$ $$I = \text{R}7\,200$$

The total amount received is the principal plus the interest earned.

$$\text{Total amount } (A) = P + I$$ $$A = \text{R}19\,200 + \text{R}7\,200$$ $$A = \text{R}26\,400$$