Answer: 18
Explanation:
We want to find out how many letters each postman can sort in an hour to make the process easier. Since we're already told that 10 postmen can sort 500 letters in 30 minutes, we can use that info to find that each postman can sort 100 letters per hour as 10 postmen would be able to sort 1000 letters in 1 hour.
Then, we can apply this info to find out how many postmen can sort 600 letters in 20 minutes. We can multiply the time and task each by 3 to find that this is the same rate as sorting 1800 letters in 1 hour, so the answer is 18 postmen.