Hello Guest!

A rectangle can be made with 2 vertical dots and 2 horizontals.

For the 2 vertical dots there are 5 to choose from.

that is choosing 2 dots from 5 or 5 choose 2.

Now applying the same logic, we also need to choose 2 horizontal dots.

There are 5 dots to choose from. so we have 5 choose 2

5 choose 2 is 10. there are 10 choices for horizontal and 10 choices for vertical. 10x10=100 choices for a rectangle