If x is the cost of one sweater, then, based on the given information you can write:
4x + 20 = 160 the cost of 4 sweaters is 4x, plus $20 is equal to $160
Solve the previous equation as follow:
4x + 20 = 160 subtract 20 both sides and simplify
4x = 160 - 20
4x = 140 divide by 4 both sides and simplify
x = 140/4
x = 35
Hence, the cost of one sweater is $35