Question

If 4/5 of a cistern is filled in 1 minute, how much time will be required to fill the empty cistern?

Answer 1

Let the capacity of cistern be 20L.
Therefore, 4/5 of 20L = 16L.
Now, 16L is being filled in 1 min.
1L will be filled in 1/16 min.
Hence, 20L will be filled in 20×1/16 min.
= 5/4 min.
= 1 min 15 sec. [Answer]

Short Method:
4/5 part is filled in 1 min.
1 part (full) will be filled in 1/(4/5) min.
= 5/4 min
= 1 min 15 sec.

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