（2/3-1/6-1/5×1/6）÷（1/5+1/6）
=(4/6-1/6-1/30)/(11/30)
=14/11;
25/2＋5/3÷4-5/7×14/5
=25/2+5/12-2
=131/12;
29/5＋21/5×17/7-17/7
=29/5+17/7(16/5)
=(29*7+17*16)/35
=95/7;
2/5÷a÷2/3=3/5 ;
a=1;
a÷1/2+1/4=2/5
2a+1/4=2/5;
a=3/40