D. 14 min. 40 sec
Feedback Part filled in 4 minutes = 4(1/15+1/20)=7/15.
Remaining part =1/15 -7/15 =8.
Part filled by B in 1 minute =1/20
1/20:8/15:: 1 : x
x = 8/15 x 1 x 20= 10 2/3min = 10 min. 40 sec.
The tank will be full in (4 min. + 10 min. + 40 sec.) = 14 min. 40 sec.