The exact size of the pipes and reservoir does not matter. If we label the large pipes L, the small pipes S and if the reservoir has a total of T litres, then:
T ÷ 6L = 12 which means L = T ÷ 72
and
T ÷ 6S = 18 which means S = T ÷ 108
we want T ÷ (6L + 6S)
= T ÷ ((6 x T ÷ 72) + (6 * T ÷ 108))
= T ÷ ((648 x T + 432 x T) ÷ 7776)
= T ÷ ((1080 x T) ÷ 7776)
= 7776 x T ÷ (1080 x T)
= 7776 ÷ 1080
= 7.2 hours
= 7 hours and 12 minutes. QED.
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A large fresh water reservoir has two types of drainage system. Small pipes and large pipes.
