Management Aptitude Test Sep 2011 Paper

(3) Part filled by (A + B) in 1 min = $\frac{1}{12}+\frac{1}{15}$ .
Part filled by (A + B ) in 5 min = 5$\left(\frac{1}{12}+\frac{1}{15}\right)$
= $\frac{9\times 5}{60}=\frac{3}{4}$
Part emptied by (A + B + C) in 1 min
Thus, time taken to empty the full tank = 60 min
= $\frac{1}{6}-\frac{1}{12}-\frac{1}{15}=\frac{1}{60}$
Hence, time taken to empty the $\frac{3}{4}$ th tank = 60 x $\frac{3}{4}$ = 45 min