AP ICET 2015 Paper

Part of tank filled in 2 hours $=\frac{1}{3}+\frac{1}{4}=\frac{17}{12}$
Part of tank remaining empty after 2 hours $=\frac{5}{12}$
Part of tank filled by B in the 3rd hour $=\frac{1}{4}=\frac{3}{12}$
Part of tank remaining empty after 3rd hour $=\frac{5}{12}-\frac{3}{12}=\frac{1}{6}$
Time taken by A to fill $\frac{1}{6}$ part of the tank
$=\frac{\left(\frac{1}{6}\right)}{\left(\frac{1}{3}\right)}=\frac{1}{2}$hr
∴ Total time taken $=2+1+\frac{1}{2}=3\frac{1}{2}$hrs