Explain it in English

Initial speed ($S_1$) = 120 km/h
Initial time ($t_1$) = 10 minutes

Convert the time to hours:

$$t_1 = 10 \text{ minutes} = \frac{10}{60} \text{ hours} = \frac{1}{6} \text{ hours}$$

Use the distance formula: Distance = Speed $\times$ Time

$$D = S_1 \times t_1$$ $$D = 120 \text{ km/h} \times \frac{1}{6} \text{ hours}$$ $$D = 20 \text{ km}$$

The motorist takes 5 minutes longer to cover the same section at the allowed speed.
New time ($t_2$) = Initial time + 5 minutes

$$t_2 = 10 \text{ minutes} + 5 \text{ minutes} = 15 \text{ minutes}$$

Convert the new time to hours:

$$t_2 = 15 \text{ minutes} = \frac{15}{60} \text{ hours} = \frac{1}{4} \text{ hours}$$

Speed limit ($S_2$) = Distance / New time

$$S_2 = \frac{D}{t_2}$$ $$S_2 = \frac{20 \text{ km}}{\frac{1}{4} \text{ hours}}$$ $$S_2 = 20 \text{ km} \times 4$$ $$S_2 = 80 \text{ km/h}$$

The imposed speed limit is:

$$\text{80 km/h}$$