1/2×1/15+1/3×1/2
=1/2×(1/15+1/3)
=1/2×(1/15+5/15)
=1/2×(6/15)
=1/2×(2/5)
=1/5怎么来的（2/5）6/15化简分子分母同除以3得到2/5