normally in a statement like yours
s=p(1+r)n
a trailing multiplier of n is not written that way
I supect you mean: s=p(1+r)^n
(1+r)^n = s/p
take 1/n th root
1+r = (s/p)^(1/n)
r = (s/p)^(1/n) - 1
Make r the subject s=p(1+r)n
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