Answer:
10 minutes
Step-by-step explanation:
Assuming men and minutes are interchangeable, 3 times as many men can be assumed to assemble the toys in 1/3 as much time.
21 men can make 21 toys in 30/3 = 10 minutes.
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Additional comment
We have to assume there are no rate-limiting steps in the process. This assumption is not always valid.
