arithmetic - taps a and b can fill a bucket in 12 minutes and 15 minutes respectively if both are opened and a is closed after 3 minutes how much further time would it take for b to fill the bucket - skillgun

Part filled in 3 min = 3[(1/12) + (1/15)] = 3 * (9/60) = 9/20
Remaining part = 1 - (9/20) = 11/20
Part filled by B in 1 min = 1/15
(1/15) : (11/20) = 1 : x or x = (11/20) * 1 * (15/1) = 8 min 15 sec
Remaining part is filled by B in 8 min. 15 sec.