Assertion (A): The area of a rectangle 4 m by 5 m is 20 m. Reason (R): Area of a rectangle = length × breadth. Choose the correct option.

**Rectangle**: area = length × breadth = $l\times b$.

**Square**: area = side × side = $s\times s=s^2$.

Units matter — both length and breadth must be in the same unit before multiplying. If $l$ is in cm and $b$ is in $m$, convert one of them first. Always write the answer with the correct square unit $({\text{cm}}^2$ or $m^2$).