依据题意列式计算如下:5/9×2/21+5/9×10/21+4/9×4/7=5/9×(2/21+10/21)+4/9×4/7=5/9×4/7+4/9×4/7=4/7×(5/9+4/9)=4/7=7分之4
5/9×2/21+5/9×10/21+4/9×4/7=5/9×(2/21+10/21)+4/9×4/7=5/9×4/7+4/9×4/7=(5/9+4/9)×4/7=1×4/7=4/7