What is the cost of one carton of milk?

Let $M$ be the cost of one carton of milk.  
Let $Y$ be the cost of one carton of yoghurt.  
From the problem:

$$
3M + 2(2M) = R24.50
$$
$$
3M + 4M = R24.50
$$
$$
7M = R24.50
$$

$$
M = \frac{R24.50}{7}
$$
$$
M = R3.50
$$
The cost of one carton of milk is **R3,50**.

Cost of one carton of milk ($M$) = $R3.50$.

$$
Y = 2 \times R3.50
$$
$$
Y = R7.00
$$

$$
\text{Total cost} = M + Y
$$
$$
\text{Total cost} = R3.50 + R7.00
$$
$$
\text{Total cost} = R10.50
$$
The cost of one carton of milk and one carton of yoghurt is **R10,50**.