This one used to involve a snail in a well, but the idea is the same. On the surface, it seems like the monkey gains 5 yards and loses 2, so he averages three per 2 seconds or 1.5 per second. That would mean 60 / 1.5 = 40.
Wrong! There's a catch: when the monkey reaches the top, he stops slipping. Let's figure out when. Remember that we are counting two seconds at a time.
10 seconds: If he averages 1.5 yards each, that totals 15. Nowhere near the top, so keep going.
20 seconds: Now he is up 20 * 1.5 = 30
30 seconds: 45 yards, still more than 15 yards away.
34 seconds: 51 yards, not quite enough for a last leap
36 seconds: 54 yards, barely out of reach for that leap
Time to slow down and study in detail what happens in next:
37 seconds: 54+5 = 59
38 seconds: 59-2 = 57
39 seconds: 57+5=62 and he is done!
