How to multiply columns and add the results?

Assume that a table has two columns named "x" and "y" filled with numbers; for example, x contains the values (1, 2, 3), and y contains (5, 8, 20).

How can I calculate the sum of the products of the columns, i.e. (1 × 5) + (2 × 8) + (3 × 20) = 81?