Eighteen pupils were given a playground to clear in 1h 40min. If six of the pupils did not turn up, how much longer did it take the rest of the pupils to clear the playground?

The original time for 18 pupils to clear the playground is 1 hour 40 minutes.

$$1 \text{ hour} = 60 \text{ minutes}$$ $$1 \text{ hour } 40 \text{ minutes} = 60 \text{ minutes} + 40 \text{ minutes} = 100 \text{ minutes}$$

The total work is the product of the number of pupils and the time taken.

$$\text{Total work} = 18 \text{ pupils} \times 100 \text{ minutes} = 1800 \text{ pupil-minutes}$$

Six pupils did not turn up, so the number of pupils working is:

$$\text{Pupils working} = 18 - 6 = 12 \text{ pupils}$$

To complete the same amount of work with fewer pupils, the time taken will be:

$$\text{New time} = \frac{\text{Total work}}{\text{Pupils working}} = \frac{1800 \text{ pupil-minutes}}{12 \text{ pupils}} = 150 \text{ minutes}$$

The difference between the new time and the original time is:

$$\text{Difference} = (2 \text{ hours } 30 \text{ minutes}) - (1 \text{ hour } 40 \text{ minutes})$$

To subtract, convert 2 hours 30 minutes to 1 hour 90 minutes:

$$\text{Difference} = (1 \text{ hour } 90 \text{ minutes}) - (1 \text{ hour } 40 \text{ minutes}) = 0 \text{ hours } 50 \text{ minutes}$$

The playground took 50 minutes longer to clear.

Comparing this to the options provided, option A contains "50min".

The final answer is $\text{50 minutes}$.