721. Find the common measure of two segments of length $1 / 5$ m and $1 / 3$ m.
$\triangleright$ A segment of length $1 / 15$ m fits three times in the first and five times in the second. $\triangleleft$

In arithmetic, a similar operation is called reducing fractions to a common denominator. The common denominator can be taken as the product of the denominators of the fractions, if nothing smaller can be found.