Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: a sum of money at simple interest amounts to rs . 815 in 3 years and to rs . 865 in 4 years . the sum is :
s . 665
750.