Cross-multiply when you're trying to solve for a variable inside of a proportion and the two fractions are set equal to each other.
Ex: [tex] \frac{10}{0.01} [/tex] = [tex] \frac{10000}{x} [/tex]
In this case you would cross multiply to get 10x = 10000(0.01)
Multiply across when the two (or more) fractions are side-by-side and the equation calls for it.
Ex: [tex] \frac{1}{4} [/tex] x [tex] \frac{5}{6} = x[/tex]
