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: what will be the difference between simple and compound interest at 6 % per annum on a sum of rs . 1000 after 4 years ?
22.5
Rs. 319.10.