Which equation is not equivalent to the formula r = st?

a.) s = r/t
b.) t = r/s
c.) s = t/r
d.) r = ts

We are given the formula r = st. We evaluate each option to know the correct answer.

A.) S = r/t 
r = st

B.) t = r/s
r = st

C.) s = t/r
r = t/s

D.) r = ts
r = st

Therefore, the correct answer among the choice given is option C. It is not equivalent to the formula.