calculate the time that will be taken by 6 large taps and 6 small taps together to drain the reservoir?

Question

There was a big pond with two taps - small tap and large tap.

Six large taps can drain the reservoir completely in exactly 12 hours while six small taps will take 18 hours to drain the water.

Now task is to calculate the time that will be taken by 6 large taps and 6 small taps together to drain the reservoir?

Answer 1

7 hours 12 minutes

Answer 2

Let's assume the reservoir holds 36 liter (lowest common multiple).
6 large taps will drain 36/12 = 3 liter per hour. 6 small taps will drain 36/18 = 2 liter per hour.
Together they drain 5 liter per hour and will empty the reservoir in 36/5 = 7.2 hours = 7 hours, 12 minutes

Answer 3

1 Large TAP will drain in 1 hr - 1/12
6 Large TAP will drain in 1 hr - 6/12 i.e. 1/2

1 Small TAP will drain in 1 hr - 1/18
6 Small TAP will drain in 1 hr - 6/18 i.e. 1/3

So 6 Large and 6 Small Tap will drain in 1 Hr - 5/6

Total Drain time - 6/5 i.e. 1 Hr 12 Minute