（1）x的2倍减去五分之二与2.5的和,差是3.5,求x.
2X - (2/5 + 2.5) =3.5
2X - 2.9 =3.5
2X = 6.4
X = 3.2
（2）12减去1.25与x的积,再除以0.4,商是21.5,求x.
( 12 - 1.25X )÷0.4 = 21.5
30 - 0.5X = 21.5
0.5X = 8.5
X = 17
(3）m的40%与12的60%的和是9.6,求m.
40%m +12×60% = 9.6
40%m + 7.2 =9.6
40%X = 2.4
X = 6