You have R18.50, your friend 1 has R12.42 and friend 2 has R15.03. You need to take a taxi that costs R50. How much more money do you need?

Round each amount to the nearest whole number.
Your money: $\text{R}18.50 \approx \text{R}19$
Friend 1's money: $\text{R}12.42 \approx \text{R}12$
Friend 2's money: $\text{R}15.03 \approx \text{R}15$
Taxi cost: $\text{R}50$

Estimated total money available:

$$\text{R}19 + \text{R}12 + \text{R}15 = \text{R}46$$

Estimated money still needed:

$$\text{R}50 - \text{R}46 = \text{R}4$$

$$\text{Total available money} = \text{R}18.50 + \text{R}12.42 + \text{R}15.03$$ $$\text{Total available money} = \text{R}45.95$$

$$\text{Money needed} = \text{Taxi cost} - \text{Total available money}$$ $$\text{Money needed} = \text{R}50.00 - \text{R}45.95$$ $$\text{Money needed} = \text{R}4.05$$

We can add the money needed to the total available money to ensure it equals the taxi cost.

$$\text{R}45.95 + \text{R}4.05 = \text{R}50.00$$

This matches the taxi cost.

You need to collect an additional $\text{R}4.05$.