80% of p = 220 (We know this as the sale price is 20% off, so there is still 80%)
0.8p = 220
p = 220/0.8 (This is because we are making p the subject of the formula)
p = 275 (If you are not allowed to use a calculator for this, then: 220/0.8 is the same as 2200/8 because we are just simply multiplying both the numerator and denominator by 10. This can be simply worked out by long division)
