tap a can fill a cylindrical container in 3 minutes. tap b can fill the same cylinder in 15 minutes. how long will it take the two taps together to fill the container?

tap A fils in one minute = 1/3 cylinder

tap B fills in one minute = 1/15 cylinder

both in one minute = 1/3 + 1/15 

   = 2/5 cylinder

both fill full cylinder in = 5/2 minute

  = 2 minute and 20 seconds