Finding cost when only the margin is given

Sometimes the information given is limited, as in this question below. Try to make equations using the information in the question. 

A store sells kettles at a markup of 18% of the selling price. The store's margin on a particular model is $6.57.

a) For how much does the store sell the kettles?

We know that 0.18S = 6.57, where ‘margin’ means contribution to fixed costs.

So: S = 6.57/0.18 = 36.5

b) What was the cost of the kettles to the store?

S = C + 0.18S and so 36.5 = C + 6.57. Therefore C = 36.5 – 6.57 = 29.93

c) What is the rate of markup based on cost?

6.57/29.93 = 22% (rounded)