Question 1163865
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Tutor @ikleyn showed an excellent informal common sense solution that gets you to the answer quickly.<br>
Here is a variation of exactly the same method -- same logical reasoning, but different calculations.<br>
The large pipe works 4 times as fast as the small pipe; so when the two together fill the pipe in 4 hours, the large pipe does 4/5 of the work and the small pipe does 1/5 of the work.<br>
Since the small pipe does 1/5 of the work in 4 hours, the number of hours it would take the small pipe alone to do the job is 4*5 = 20 hours.<br>