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Option 5 : 11.25 minutes

Given:

Pipe C = 50% + Pipe A

Pipe B: Pipe C = 4 : 5

Pipes opened every 5 minutes

Formula:

If time taken by pipe A to fill tank is ‘a’ minutes and by pipe B is ‘b’ minutes then total time taken to fill the tank t ⇒ (1/t) = (1/a) + (1/b)

Calculation:

Let, time taken to fill the tank by pipe A = X

⇒ Pipe C = 0.5X + X

⇒ Pipe C = 1.5X

⇒ Pipe B: Pipe C = 4: 5

⇒ Pipe B = 4 × 1.5X/5

⇒ 1/ 4 = (1/ X) + (5/6X) + (1/1.5X)

⇒ 1/ 4 = (6 + 5 + 4)/(6X)

⇒ 1/ 4 = 15/ 6X

⇒ 6X = 60

⇒ X = 10 minutes

⇒ Time taken by pipe C = 15 (∵ 1.5 × 10 = 15)

⇒ Time taken by pipe B = 6 × 10/5

⇒ Time taken by pipe B = 12 minutes

Calculating units/ minutes for each pipe

LCM for 10, 12 and 15 = 60

⇒ Pipe A = 6 (∵ 60/10 = 6)

⇒ Pipe B = 5 (∵ 60/12 = 5)

⇒ Pipe C = 4  (∵ 60/15 = 4)

Total quantity to be filled = 60 unit

⇒ Units filled by pipe A = 30 (∵ 6 × 5 min = 30)

Remaining Unit = 60 - 30 = 30

⇒ Units filled by pipe B = 25 (∵ 5 × 5 min= 25)

Remaining Unit = 30 - 25 = 5

⇒ Units filled by pipe C = 5 (∵ 4 × 5/4 min= 5)

⇒ Time taken = 5 + 5 + (5/4)

⇒ Time taken = 11.25 minutes

∴ Time taken to fill up the tank is 11.25 minutes

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