Intelligent trader with 3 sacks and coconuts

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 check point he has to give one coconut for each sack carrying.
How many coconuts are left in the end?


Reminder: He is an intelligent trader :)
This puzzle is courtesy Chaitali Jain, shared on our facebook page.

Answer with Solution

Answer: He will be left with 1 sack containing 25 coconuts.
How?
As he has to give one coconut for every sack he carries, he will try to get rid of the sacks as early as possible.
To get rid of the first sack:
He should be able to fill coconuts from one sack to other two sacks, lets say he is able to do it after x checkpoints, to get x we can say

(space left in first sack)x + (space left in second sack)x = 
(remaining coconuts in third sack)30 -x;
x = 10;

So after 1st checkpoints, he will be left with 2 sacks containing 30 coconuts each and he will throw the third sack.
To get rid of the second sack:
He should be able to fill coconuts from second sack to first sack, lets say he is able to do it after another y checkpoints, to get y we can say

(space left in first sack)y = (remaining coconuts in second sack)30 -y;
y =15

Thus after 25th check point he will be left with one sack containing 30 coconuts, at this point he will throw away the second sack as well.
Now we can easily say that after 30th checkpoint he will be left with 30 -5 = 25 coconuts.

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