A monkey start climbing up a tree 21 ft tall.Each hour it hops 3 ft and slips back 2 ft. How much time would it take the monkey to reach the top.  Choose the correct choice from the following choices.
A) 15 hrs	B) 18 hrs	C) 19 hrs	D) 17 hrs	E) 16 hrs

Rationale: "if monkey hops 3 ft and slips back 2 ft in a hour , it means the monkey hops ( 3 ft - 2 ft ) = 1 ft / hr . similarly in 18 hrs it wil be 18 ft . but since the height of the tree is 21 ft , so if the monkey hops up the tree in the next hr i . e 19 th hr then it reaches at the top of the tree . hence it takes 19 hrs for monkey to reach at the top answer : c"
Answer: C