Question 616279
Work out the area before and after by multiplying the length by the width for each room size.

So for the smaller room:

a = 21 x 18
  = 378 sq ft

and for the larger room

A = 24 x 20
  = 480 sq ft

So the difference is the area of the larger room minus the area of the smaller room:

Area difference = A - a
                = 480 - 378
                = 102 sq ft