Christine L. answered 03/04/15
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HS Math Teacher and HS/College Tutor specializing in Math & Test Prep
Your question is slightly vague.
If the $60 fee is required of everyone whether they only want 1 wash or a million, the formula will be like this:
Let n = number of visits
$60 + $15n
That is because everyone will be charged 60 as a flat fee, and then 15n represents the actual amount that you must pay everytime you visit.