I go to market i forget carrying money with me . I met with my two friends there i borrowed 1000 Rs from A friend 500 Rs from B friend . I have 1500 Rs i lost 1000 Rs from 1500 i have 500 i shop 300 from 500 i left 200 i give 100 Rs to A friend from 200 100 to B friend i have to pay back 900 insted of 1000 to A friend also pay back 400 instead of 500 to B friend total to pay back is 900. now it becomes 900 + 400 = 1300 + 300 (shopping) = 1600 but i borrowed 1500 so how 1600 is left

Answer :

Borrowed money from 
A  =  Rs 1000 
B  = Rs 500 

Total borrowed money   =  Rs 1500 

And we know 

Total borrowed money  =  total money spent  + remaining money
SO
First he lost Rs.1000 and then he shop for Rs 300 
 

So total money spent  =  Rs . 1300

And remaining money =  Rs 200 

SO,

1500  =  1300  +  200 

Now he gave 100 to A and 100 to B 

So,

1500 -  100  - 100  =  1300 + 200 - 100 - 100 

1300 =  1300 

SO, Here we can't add 300 to that money , we add 200 and get

1500  = 1500 

The error in your calculation is the last  step where you add 300 to 1300 , but instead of that you can only add 200 ( the amount you have )

Than you can achieve 
borrowed money =  total money spent