A monkey climbs 20 feet at the beginning of each hour and rests for a while when he slips back 5 feet before he again starts climbing in the beginning of the next hour. So, the resultant height moved in an hour will be 15 feet.

In six hours, the monkey reaches 90 feet. At the start of 7th hour, monkey climbs 20 feet. With this it touches 110 feet.

So, at exactly 3:00 PM monkey reaches 110 ft for the first time.