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Option 3 : 690/29

Given:

Pipe A, B are filling pipes that can fill the tank in 45 and 54 minutes.

C can empty the tank in 72 Minutes.

A, C are kept open for 30 minutes, then pipe B and C are opened for 24 minutes.

Calculation:

Let us take the LCM of the time of the three pipes i.e.

LCM of (45, 54, 72) minutes = 1080 units.      ----(1)

Let 1080 units be the capacity of the tank.

⇒ Efficiency of A = (1080/45) = 24 units

⇒ Efficiency of B = (1080/54) = 20 units

⇒ Efficiency of C = (1080/72) = -15 units

(∵ A, B are filling pipes so their efficiency is positive and as C is an empty pipe, its efficiency is negative)

Work was done By A, C in 30 minutes = (24 - 15) × 30 = 270 unit      ----(2)

Now, B, C was opened for 24 minutes,

So, work done by B, C in 24 minutes = (20 - 15) × 24 = 120 units      ----(3)

From (1), (2), (3) we get the left work = 1080 - 270 - 120 = 690 unit

Time taken by all three pipes to fill the Remaining tank (690)/(24 + 20 – 15) = 690/29 min.

∴ The time taken by all pipes to fill the remaining tank is 690/29 min.

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