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Option 3 : 5 hours

Given, tap A is 40% more efficient than tab B

∴ The ratio of efficiency of A : B = 7 : 5

Total time taken by A alone to fill the tank = 4D hours

∴ Total time taken by B alone to fill the tank = 7/5 × 4D hours = 5.6D hours

Time taken by B alone to fill 3/4th of the tank = 4.2D hours

According to the question,

4.2D = D + 16

⇒ D = 5 hours

∴ Total time taken by A alone to fill the tank = 4 × 5 = 20 hours

Total time taken by B alone to fill the tank = 5.6D = 28 hours

Let the total capacity of the tank be LCM of (20 and 28) = 140

Number of units filled by A in 1 hour = 140/20 = 7

Number of units filled by B in 1 hour = 140/28 = 5

Let the number of hours for which A and B didn’t work with their full efficiency be ‘x’ hours

Now, according to the question,

⇒ x + × 10 = 140

⇒ 4x + 10 × 12 = 140

⇒ x = 5 hours
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