Answer: For this kind of questions, you are using simple interest, right?. Here will be the answer
Formula of Simple Interest is....
SI= principle x rate x time
1) Principle (money): 800
Rate: divide 31 by 100, it will be 0.31
Time: 5 years
2) Equation: 800 x 0.31 x 5
3) You get the final answer of... 1240
Hope this helped. Remember... know the formula for these kind of situations :)