The first 3 common multiples of 5 and 15 are 15, 30, and 45. Remember that the number itself is also a multiple of any number, since you can multiply it by 1.
A rectangle is a quadrilateral with all four angles right angles. It follows form this that the opposite sides are parallel and of the same length. A square is a quadrilateral with all four angles right angles and all four sides of the same length.
Also note that a square is always a rectangle, but a rectangle is not always a square.
