In order to calculate this, we can use this solution:
If it is a loss:
Cost Price = Selling Price × (100 ÷ (100 - Perc. Loss))
If it is an increase:
Cost Price = Selling Price × (100 ÷ (100 + Perc. Increase))
a) £54.60 × (100 ÷ (100 - 9))
£54.60 × (100 ÷ 91) = £60
b) £34.40
c) £28.50
d) £64
e) £12.50