Cross multiplication is a math process used to solve for a missing number when comparing two equal ratios. It involves multiplying the top and bottom numbers of each ratio or fraction with the one opposite to it in the other ratio.

### Multiply first numerator and second denominator

The first numerator is the top of the ratio on the left of the equal sign. The second denominator is the bottom number on the right of the equal sign. In the formula 1/2 = n/4, multiply 1 x 4. The result is 4.

### Multiply second numerator and first denominator

Multiply the numerator, or top number, in the ratio on the right side of the equal sign by the denominator, or bottom number, in the ratio on the left side of the = sign. In the example, multiply n x 2. The result is 2n.

### Set up the equation and solve for n

To complete the algebraic equation and solve for n, state the formula 2n = 4. Each side of the equal sign has one of the results of the cross multiplication steps. To solve for n, divide each side by 2. Since 2n/2 is n, and 4/2 is 2, n = 2.