a) As a fraction of the original price Louisa will pay ...
0.85c . . . . 85% of the subscription price
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b) Louisa's expression (-0.15c) tells the amount of the discount, not the cost of the subscription. If Louisa were to write the cost of the subscription, her expression would be ...
c - 0.15c
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c) The expression of (a) is the cost of the subscription; the expression Louisa wrote is the amount of cost reduction due to the discount.
