A **rate** is a ratio involving quantities, typically with different units.

A rate where the second quantity is 1 is called a **unit rate**.

The ratio is $\frac{1}{2}$ pound of butter per $1\frac{1}{2}$ cups of flour.

To find the amount of butter per 1 cup of flour, or the unit rate, divide $\frac{1}{2}$ pound of butter by $1\frac{1}{2}$ cups of flour.

Use the improper fraction $\frac{3}{2}$ in place of the mixed number $1\frac{1}{2}$.

$\begin{array}{rcl}\frac{\frac{1}{2}}{\frac{3}{2}}& =& \frac{1}{2}\xf7\frac{3}{2}\\ & =& \frac{1\times 2}{2\times 3}\\ & =& \frac{2}{6}\\ & =& \frac{1}{3}\end{array}$

So, $\frac{1}{3}$ pound of butter is needed for 1 cup of flour.