4/9÷10/7+5/9*7/10
=4/9*7/10+5/9*7/10=7/10*(4/9+5/9)=7/10
0.8*3/8*1/9
=0.3*1/9
=1/30
（1/2-1/5+7/12）*60
=30-12+35
=65-12
=53
1/21+3/4*2/3÷2
=1/21+1/2*1/2
=1/21+1/4
=25/84
（1/2-5/11）*（5/9+2/13）
=(11/22-10/22)*(65/117+18/117)
=1/22*83/117
=83/2574