When to multiply across or cross multiply?

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:  = 

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:  x 

1/2 a lawn per hour

Step-by-step explanation:

2÷4= 0.5