a) £350 * (100% - 20%) / 100% = £350 * 0.8 = £280 -- sale price
b) £68 / (100% - 20%) * 100% = £68 / 0.8 = £85 -- original price
a) £350 * (100% - 20%) / 100% = £350 * 0.8 = £280 -- sale price
b) £68 / (100% - 20%) * 100% = £68 / 0.8 = £85 -- original price