Nối:

Giải thích

\[\frac{{27}}{{27}} = 1\]                            \[\frac{{15}}{{21}} = \frac{{15:3}}{{21:3}} = \frac{5}{7}\]                                               \[\frac{{18}}{{36}} = \frac{{18:18}}{{36:18}} = \frac{1}{2}\]