Answer with Explanation:
Let B be turned off after x minutes. Then,
Part filled by (A + B) in x min. + Part filled by A in (30 -x) min. = 1.
Therefore,x (2/75 + 1/45) + (30 - x). 2/75 = 1
=> 11x/225 + (60 -2x)/75 = 1
=> 11x + 180 - 6x = 225.
=> x = 9.