How many square inches are in a 10 inch by 5 inch rectangle?

50

Get a ruler and draw them. Then you can count them. You will get five identical rows of five squares, each one square inch. The count is 50. Or you can use the shorthand 5 x 10 = 50. Since we live with numbers, you make up a table with numbers from 1 to any convenient upper limit down the side and likewise across the top. It looks like this:

.. 2 3 4 ...
1 2 3 4
2 4 6 8
3 6 9
4 8
5
6
7
... and so on. Fill in the spaces with the product of the two numbers and then memorize all of them.