J varies inversely with v (or "J and v are inversely proportional") if there is a constant k such that

J = k/v

First we will find the constant k

J = 90 when v = 9 means

90 = k/9

k = 90*9

k = 810

now we know the equation for J and v

J = 810/v

when v = 45 for J we have

J = 810/45

J = 18.