Multiple choice

Pipe A can fill a tank in 8 hours and pipe B can fill it in 6 hours. If they are opened on alternate hours, with pipe A opened first, in how many minutes will the tank be full?

  1. 420

  2. 450

  3. 390

  4. 400

  5. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A fills 1/8 per hour, B fills 1/6 per hour. In 2 hours, they fill 1/8 + 1/6 = 7/24. In 6 hours (3 cycles), they fill 21/24. Remaining 3/24 = 1/8. Pipe A fills 1/8 in 1 hour. Total time = 6 + 1 = 7 hours = 420 minutes.

AI explanation

By finding the least common multiple of 8 and 6, the total capacity of the tank is assumed to be 24 units. Pipe A fills 3 units per hour and pipe B fills 4 units per hour; when opened on alternate hours starting with A, they fill 7 units every two hours. After 6 such cycles, taking 12 hours, 21 units of the tank are filled, leaving exactly 3 units empty. Pipe A is opened next and fills the remaining 3 units in exactly 1 hour, making the total filling time 13 hours. Converting this total time to minutes yields 13 multiplied by 60, resulting in 420 minutes.

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