Three fill pipes A, B and C can fill separately a cistern in 12, 16 and 20 minutes respectively. A was opened first. After 2 minute, B was opened and after 2 minutes from the start of B, C was also opened. Find the time when the cistern will be full after opening of C?
Answer with explanationAnswer: Option A
Let cistern will be full in x min. Then part filled by A in x min + part filled by
B in (x-2) min + part filled by C in (x-4) min = 1
⇒ x/12 + (x-2)/16 + (x-4)/20 = 1 ⇒ 47x – 78 = 240⇒ x = 162/47 = 3 21/47 min
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