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.
1. £600 + (6% of £600 × 3)
£600 + (36 × 3)
£600 + £108 = £708.
2. £1800
3. £981
