Problem:

Calculating the total sales price of all the items listed in column A, based on the amount of items sold (column B), and the price per unit (column C).

Solution 1:

Use the SUMPRODUCT function as shown in the following formula:
=SUMPRODUCT(B2:B5,C2:C5)

Solution 2:

Use the SUM function as shown in the following Array formula:
{=SUM((B2:B5)*(C2:C5))}