91.53=81x1.13, which is a 13% increase on 81. So a=81 and b=13%?
If the sum is over a number of days (not specified in the question), then the percentage will be different, but the quantity 81 stays the same:
7 days: 1.76% per day
6 days: 2.06% per day
5 days: 2.47% per day
4 days: 3.1% per day
3 days: 4.16% per day
2 days: 6.3% per day
1 day: 13%