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Problem 385

Algebra Difficulty 2.5 Multiple choice CEMC Gauss (Grade 8) · Canada · 2015

Two puppies, Walter and Stanley, are growing at different but constant rates. Walter’s mass is 12 kg and he is growing at a rate of 2 kg/month. Stanley’s mass is 6 kg and he is growing at a rate of 2.5 kg/month. What will Stanley’s mass be when it is equal to Walter’s?

Pick one

Official solution

The rectangle has area $5\times12=60$ cm $^2$ .

Each of the two congruent unshaded triangles has area $\frac12\times2\times5=5$ cm $^2$ .
The area of the shaded region is equal to the area of the rectangle minus the areas of the two unshaded triangles, which is $60-5-5=50$ cm $^2$ .

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.