Sell price = $15.50
Cost = $6
Multiply the cost by the percentage markup (1 +x ) , that equation must be equal to the selling price.
6 (1 + x ) = 15.50
Solve for x
6 + 6x = 15.50
6x = 15.50-6
6x = 9.5
x= 9.5/6
x= 1.583
Multiply by 100
1.583 x 100 = 158.3%