Rút gọn phân số: \(\displaystyle{{18} \over {30}};{{25} \over {40}};{{42} \over {72}};{{80} \over {100}}\).
\(\displaystyle {{18} \over {30}} = {{18:6} \over {30:6}} = { 3\over 5}\)
\(\displaystyle {{25} \over {40}} = {{25:5} \over {40 :5}} = { 5\over 8}\)
\(\displaystyle {{42} \over {72}} = {{42:6} \over {72:6}} = { 7\over 12}\)
\(\displaystyle {{80} \over {100}} = {{80:20} \over {100:20}} = { 4\over 5}\)
-- Mod Toán lớp 4