An intelligent trader travels from one place to another carrying 3 sacks having 30 coconuts each. No sack can hold more than 30 coconuts. On the way, he passes through 30 checkpoints and on each checkpoint, he has to give one coconut for each sack he is carrying. How many coconuts are left in the end?

For the 1^st 10 checkpoints, he has to give 30 coconuts, 1 out of each bag. Having crossed the 10^th checkpoint, he is left with 90 - 30, i.e. 60 coconuts with 20 in each bag. He empties out contents of 1 bag, puts 10 coconuts each in the other 2 bags and leaves the 3^rd empty bag behind. He now has 60 coconuts in 2 bags with 30 each. He now crosses another 15 checkpoints, giving 2 coconuts at each point. Having crossed the 25^th checkpoint, he is left with 60 - 30, i.e. 30 coconuts with 15 in each bag. He empties the contents of 1 bag into the other and leaves the empty bag behind. He now has 1 bag with 30 coconuts. He now crosses the last 5 checkpoints giving 1 coconut at each. Having crossed the 30^th and the last checkpoint, he is left with 30 - 5 = 25 coconuts.