By what fraction selling price must be multiplied to get the cost price, if the loss is 20% = ?

$$\eqalign{ & \% {\text{ Loss }} = 20\% \cr & 20\% = \frac{1}{5} \cr} $$
It Cost price = 5 units
Loss = 1 units
Selling price = 5 - 1 = 4 units
Selling price = 4
we make it equal to Cost price that is (5)
So, we multiply Selling price by
$$\eqalign{ & = \frac{5}{4} \cr & {\text{Selling price}} = 4 \times \frac{5}{4} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 5{\text{ }} \cr} $$