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=10/271/4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=8/55/8x=15/16x=15/16÷5/8x=1...