Concept: The question tests on Rates(Time and Work)
It is given that Jim climbs 3 feet in 1 minute and slips down a foot in the next.
So every 2 minutes, he climbs 2 feet.
Here comes the twist. When he reaches 28 feet, he has taken 28 minutes and the last action was that he had slipped. His next action in the 29th minute would be to climb 2 m and since he climbs 3 m in the next minute, he would reach the mango in the 29th minute.