The equation used to find the total number of shells the diner collects is; S = 3(6) + 1(5).
It follows from the task content that the 6 customers each buys a Crispy Star for 3 shells and a glass of milk for 1 shell except for one who has a coupon for a free glass of milk.
Hence, the total number of shells collected by the diner is; S = 3(6) + 1(5).
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