A doctor starts a course of medicine at 72 mg on day 1 and reduces the dose by 4 mg every day.

On which day is the dose 32 mg?

a = 72, d = -4. Solving 72 + (n - 1)(-4) = 32 gives 4(n - 1) = 40, so n - 1 = 10 and n = 11. The value 10 is the number of reductions made, not the day number; the 18th day is where the dose would reach 0, not 32 mg.