A. 12 min 30 sec
B. 14 min 40 sec
C. 10 min 20 sec
D. 11 min 45 sec
Explanation:
Part filled in 4 minutes = 4(1/15 + 1/20) = 7/15
Remaining part = 1 – 7/15 = 8/15
Part filled by B in 1 minute = 1/20
1/20 : 8/15 :: 1 ; x
x = 8/15 * 1 * 20 = 10 2/3 min = 10 min 40 sec.
The tank will be full in (4 min. + 10 min. 40 sec) = 14 min 40 sec.