Triangle ABC and triangle PQR are similar. The circle drawn through A, B and C has radius 7 cm, and the circle drawn through P, Q and R has radius 4 cm.
Find area(ABC) : area(PQR).

In similar triangles every corresponding length scales by the same factor, so the circumradii are in the ratio of the sides, 7 : 4. Area ratio = ${\left(\frac{7}{4}\right)}^2=\frac{49}{16}$, i.e. 49 : 16. The option 7 : 4 is the length ratio left unsquared, 14 : 8 is that same length ratio doubled, and 16 : 49 swaps the two triangles.