Tap A can fill up an empty tank in 3 hours.Tap B can fill up the same empty tank in 4 hours.Tap C can fill up the same empty tank in 6 hours

Here's another way I like to do this kind of problem

Add

1/3 + 1/4 + 1/6 =

4/12 + 3/12 + 2/12 =

9/12     Now, take the reciprocal of this fraction 

12/9 =

4/3 =

1 + 1/3 hrs

If the volume of the tank is, say, V litres, then

A fills it at a rate of V/3 litres/hour

B fills it at a rate of V/4 litres/hour

C fills it at a rate of V/6 litres/hour

So A+B+C would fill it at a rate of V/3 + V/4 + V/6 = 3V/4 litres/hour

Time taken = V/(3V/4) = 4/3 hours  or 1hour 20 minutes. 

Thanks you for submitting this one anonymous.

I have saved it in the sticky thread "Puzzles".