A man starts climbing a 11 m high wall at 5 pm. In each minute he climbs up 1 m but slips down 50 cm. At what time will he climb the wall?

Answer: Option B

1^st Method:
Man climbs 1m and slips down 50 cm (0.5m) in one minute
i.e. he climbs (1 - 0.5 = 0.5 m) in one minute.
But in the last minute he will be climbing 1m as he gets on the top so no slip.
Time taken to climb 11 meter = $$\frac{10}{0.5}+1$$  = 21 minutes.
He climbs the wall at 5:21 pm

2^nd Method (short-cut):
$$=\frac{\text{hight}\text{of}\text{pole}-\text{slipped}\text{distance}}{\text{climbed}\text{distance}-\text{slipped}\text{distance}}$$
$$\begin{array}{rl}& =\frac{x-z}{y-z}\times t\\ & =\frac{11-0.5}{1-0.5}\times 1\\ & =21\text{minutes}\end{array}$$