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Option 3 : 30 minutes

Let the inlet taps be A and B, and outlet taps be C and D.

As, Tap A can fill the tank in 10 minutes

⇒ Part of tank filled by A in 1 minute = 1/10

Also, Tap B can fill the tank in 15 minutes

⇒ Part of tank filled by tap B in 1 minute = 1/15

⇒ Part of tank filled by Tap A and B in 1 minute = (1/10) + (1/15) = 1/6

Now, Tap C can empty the tank in 12 minutes

⇒ Part of tank emptied by tap C in 1 minute = 1/12

Also, Tap D can empty the tank in 20 minutes

⇒ Part of tank emptied by tap D in 1 minute = 1/20

⇒ Part of tank emptied by tap C and D in 1 minute = (1/12) + (1/20) = 2/15

⇒ Effective filling of tank in 1 minute when all the taps are open = (1/6) – (2/15) = 1/30

∴ The tank will be filled in 30 minutes
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