Problem 12 · The shaded rectangle
The neighbouring areas give away the sizes: height $7-2$, width $11-3-2$.
Integer answer, at most 4 digitsWhat is the area of the shaded rectangle?
Copa Cangur · SCM
Easy
Closed answer
Reasoned solution
Key idea: deduce the white rectangles' dimensions from their area and one known side.
Left rectangle: width $3$ and area $21$ → height $21/3 = 7$. Bottom strip: height $3$ and area $33$ → width $33/3 = 11$.
The grey rectangle stops $2$ cm below the top of the left rectangle: height $7 - 2 = 5$. And the bottom strip sticks out $2$ cm on the right: width $11 - 3 - 2 = 6$.
$$A = 5 \cdot 6 = 30\ \text{cm}^2.$$
Answer: 30