I've already answered 2 and 3 so I'll help with the rest.
1. We first find the percentage of the number and then add/subtract that onto the number.
a) £20 + (15% of £20) £20 + £3 = £23
b) £396 c) £810 - (5% of 810) £810 - £40.50 = £769.50
d) £589 e) £936
I cannot help with 4, as I do not know if it is increasing or not.
Problems
We find the interest and add it onto the original amount. Interest is equal to a percentage of the original amount times the number of months or years.
