Multiple choice

Pipes A, B and C can fill a tank in 30 h, 40 h and 60 h, respectively. Pipes A, B and C are opened at 7 a.m., 8 a.m. and 10 a.m., respectively on the same day. When will the tank be full?

  1. 10 : 00 pm

  2. 10 : 20 pm

  3. 9 : 20 pm

  4. 9 : 40 pm
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let the tank capacity be 120 units. A fills 4 u/h, B fills 3 u/h, C fills 2 u/h. By 10 a.m., A has worked 3 hours (12 units) and B has worked 2 hours (6 units). Total filled = 18 units. Remaining = 102 units. Combined rate = 4+3+2 = 9 u/h. Time = 102/9 = 11 hours 20 minutes. 10 a.m. + 11h 20m = 9:20 p.m.

AI explanation

Using the least common multiple of 30, 40 and 60 as the tank's capacity, the total work required is 120 units. The hourly filling rates for pipes A, B and C are 4, 3 and 2 units respectively. Between 7 a.m. and 8 a.m., only pipe A works, filling 4 units; between 8 a.m. and 10 a.m., pipes A and B work together, filling 7 units each hour for a total of 14 units. By 10 a.m., 18 units are filled, leaving 102 units to be filled by all three pipes working together. Their combined hourly rate is 4 + 3 + 2 = 9 units, so filling the remaining 102 units takes 102 divided by 9 hours, which is 11 hours and 20 minutes. Adding this duration to 10 a.m. results in a completion time of 9:20 p.m.

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