(1/3+1/5)×15
=1/3×15+1/5×15
=5+3
=8
(1/9+1/3)×27
=1/9×27+1/3×27
=3+9
=12
(1/6+1/4)×36
=1/6×36+1/4×36
=6+9
=15
89×7/88
=(88+1)×7/88
=7+7/88
=7又7/88