Speed problem

oks wrote: ↑

Sat Aug 08, 2009 3:54 pm

12. A snail, climbing a 20 feet high wall, climbs up 4 feet on the first day but slides down 2 feet on the second. It climbs 4 feet on the third day and slides down again 2 feet on the fourth day. If this pattern continues, how many days will it take the snail to reach the top of the wall?
(a) 12
(b) 16
(c) 17
(d) 20
(e) 21

OA is C.

Thanks!!

It’s obvious that every two days the snail climbs 2 feet; however, it doesn’t take the snail 20 days to climb the 20-foot wall. That is because on the last day, when the snail climbs 4 feet to reach the top of the 20-foot wall, it would not slide down again. Therefore, we need to subtract 4 from 20 first to obtain 16. For the first 16 feet, it’s true that it will take the snail 16 days to climb it. Then, it will take one more day to climb the final 4 feet, so it will take the snail a total of 17 days to climb the 20-foot wall.

Answer: C