Two circles have diameters in the ratio 1:3. Find the ratio of their circumferences.
The ratio of any two corresponding lengths of two similar figures will be the same, no matter which two corresponding lengths are chosen. So if the diameters are in the ratio 1:3, the circumferences will also be in the ratio 1:3.
(Note, though, that the areas will be in the square of this ratio; so they'll be in the ratio 1:9.)