64×2/7+65×5/7=64×2/7+64×5/7+1×5/7=64×(2/7+5/7)+5/7=64×1+5/7=64+5/7=64又5/7
=64乘(2/7+5/7)=6.4乘1=6.4
64×2/7+65×5/7=(65-1)×2/7+65×5/7=65×2/7+65×5/7-2/7=65×(2/7+5/7)-2/7=65-2/7=455/7-2/7=453/7