The length of the rectangle is 84 cm. How much the area of the rectangle will decrease if its width is reduced by 5 cm.

The area of a rectangle is determined by the product of its length and width. Let’s take that the initial width of the rectangle is a cm. Then the area S1 will be S1 = 84 * a cm square. Now we will find the area of the rectangle if we reduced its width by 5 cm. Now its width will be a-5. Let us now find its area S2 = 84 * (a-5) = 84 * a-420. We see that the expression 84 * a = S1 is the area of the original rectangle. S2 = S1 420. The formula shows that the original area has decreased by 420 square cm.
Answer: the area has decreased by 420 square cm.