The answer is 36.

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Fraction of the bath filled by both the taps in 1 minute= $\dfrac {1}{12}+ \dfrac {1}{30} = \dfrac {1}{12}$

The waste pipe can empty as much of the bath in 12 minutes that the two taps can fill in 4 minutes

The taps can fill the bath tub in 12 minutes, therefore they can fill $\dfrac {1}{3}$ of the tank in 4 minutes.

Hence, This implies, The waste pipe can empty $\dfrac {1}{3}$ of the bath in 12 minutes

Therefore it can empty the complete tank in $12 \times 3 = 36$ minutes.