a:b c:d The ratios have to stay in proportion. So if we multiply a and b by a certain number they will equal c and d. We just don't know what that number is yet. Call it something.
a* something = c and b* something =d
5 * something = x
15* something = 3
Solve for the unknown number we are multiplying by
Divide each side by 15
15/15 * something =3/15
something = 1/5
Now that we know what we are multiplying by, we can solve for x
