Let the volume of tank be 90 litres. 1st tap empty it in 30 min => Rate of 1st tap = $$frac{90}{30}$$ = 3 $$l/min$$ 2nd tap can empty it in 45 minutes. => Rate of 2nd tap = $$frac{90}{45}$$ = 2 $$l/min$$ Rate at which both taps empty it = 3+2 = 5 $$l/min$$ => Time needed to empty tank = $$frac{90}{5}$$ = 18 min