To solve a proportion like a/b = c/d, cross multiply by multiplying a × d and b × c
Set the products equal: a × d = b × c
If one fraction has an unknown, multiply across first, then solve for the unknown
Example: x/4 = 3/8
Cross multiply: 8x = 12
Solve for x: x = 12/8 = 3/2
If needed, simplify the result
Check by substituting the value back into the original proportion