ratio 1:3 can be written as 1/3
sale price / original price : 1/3 = x / 81
this is a proportion, so we cross multiply
(3)(x) = (1)(81)
3x = 81
x = 81/3
x = 27 <== sale price
and notice that 1/3 = 27/81...these are equivalent fractions because that is what proportions are, equivalent fractions
