Multiple choice

There are three pipes A, B & C that can fill a tank in 30, 40 and 60 hours respectively. A is opened at 7:00 AM, B at 8:00 AM and C at 10 AM, at what time the tank will be filled.

  1. 9: 20 PM

  2. 10: 00 PM

  3. 9: 30 PM

  4. 12: 30 PM
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A fills at 1/30 per hour, B at 1/40 per hour, C at 1/60 per hour. By 10 AM, A worked 3 hours: 3/30 = 1/10, B worked 2 hours: 2/40 = 1/20. Total filled = 1/10 + 1/20 = 3/20. Remaining = 17/20. All three together fill at 1/30 + 1/40 + 1/60 = (4+3+2)/120 = 9/120 = 3/40 per hour. Time for remaining: (17/20)/(3/40) = 34/3 = 11.33 hours = 11h 20m. Tank fills at 10 AM + 11h 20m = 9:20 PM.

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