A tap can fill a tank in 8 hours. After half the tank is filled, four more similar taps are opened. What is the total time taken to fill the tank completely?

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  1. 4 hours, 48 min
  2. 2 hours, 40 min
  3. 3 hours, 30 min
  4. 5 hours, 30 min

Answer (Detailed Solution Below)

Option 1 : 4 hours, 48 min
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Detailed Solution

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Given:

A tap can fill a tank in 8 hours.

After half the tank is filled, four more similar taps are opened.

Formula used:

Rate of filling = Work done per hour

Time = Work ÷ Rate

Calculation:

Time taken by 1 tap to fill the tank = 8 hours

Rate of 1 tap = 1 ÷ 8 (tank/hour)

Time to fill half the tank with 1 tap = 1 ÷ 2 × 8 = 4 hours

Remaining tank to be filled = 1/2

With 5 taps (1 original + 4 additional):

Rate of 5 taps = 5 × (1 ÷ 8) = 5/8 (tank/hour)

Time to fill the remaining half tank:

⇒ Time = Work ÷ Rate

⇒ Time = (1/2) ÷ (5/8)

⇒ Time = 8/10 = 0.8 hours

⇒ 0.8 × 60 = 48 minutes

Total time to fill the tank:

⇒ 4 hours (first half) + 0.8 hours (second half)

⇒ 4 hours + 48 minutes

∴ The correct answer is option (2): 4 hours, 48 minutes.

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