1/3乘以7除以1/3
=1/3除以1/3乘以7
=1乘以7
=7
（7/9+7/6）乘以4/21
=7/9乘以4/21+7/6乘以4/21
=4/27+2/9
=4/27+6/27
=10/27
1/4除以5/8+3/4除以5/8
=1/4×8/5+3/4×8/5
=（1/4+3/4）×8/5
=1×8/5
=8/5
5/8x=15/16
x=15/16÷5/8
x=15/16×8/5
x=3/2
4x+1/3=3/5
4x=3/5-1/3
4x=4/15
x=1/15
4x-1/5x=1/8
160x-8x=5
152x=5
x=5/152