To find out how much you have to pay for 24 ounces of the sandwich spread, you first need to convert ounces to pounds. There are 16 ounces in a pound.
So, to convert 24 ounces to pounds:
\[ \text{Pounds} = \frac{24 \text{ ounces}}{16 \text{ ounces per pound}} = 1.5 \text{ pounds} \]
Next, you can calculate the cost for 1.5 pounds at a rate of $6.40 per pound:
\[ \text{Cost} = 1.5 \text{ pounds} \times $6.40 \text{ per pound} = $9.60 \]
Therefore, you will have to pay $9.60 for 24 ounces of the spread.