a/b =c/d Find d

We want to solve for d. This means we want d alone on one side.

$$\begin{array}{lc} \frac{a}{b} =\frac{c}{d}&\qquad\qquad\mbox{Multiply both sides by d}\\ \frac{a}{b}\times d=c&\qquad\qquad\mbox{Multiply both sides by b}\\ a \times d = c \times b&\qquad\qquad\mbox{Divide both sides by a}\\ d = c \times b \div a \end{array}$$

And that's it!