Coconut & sacks(bag) puzzles !!

Problem statement:
An intelligent trader travels from 1 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 1 coconut for each sack he is carrying.
Maximum how many coconuts he can save when he reaches last check point ?

Ans: He will be left with 25 coconuts in 1 sack

1. First 10 checkpoints = 30 coconuts. Thus, finishing his first sack.
2. Next 15 checkpoints = 30 coconuts. Thus, finishing his second sack.
3. Last 5 checkpoints = 5 coconuts. Thus, he still has 25 coconuts in his last sack