Ta có : \(\dfrac{{16}}{{51}} < \dfrac{{17}}{{51}} = \dfrac{{17:17}}{{51:17}} = \dfrac{1}{3}\).
Lại có : \(\dfrac{1}{3} = \dfrac{{1 \times 30}}{{3 \times 30}} = \dfrac{{30}}{{91}} < \dfrac{{31}}{{90}}\)
Do đó ta có : \(\dfrac{{16}}{{51}} < \dfrac{1}{3} < \dfrac{{31}}{{90}}\).
Vậy : \(\dfrac{{16}}{{51}} < \dfrac{{31}}{{90}}\).