(1)12/7 -(5/7+0.5)
=(12/7-5/7)-0.5
=1-0.5
=0.5
(2)[1-(0.25+3/8)]÷1/4
=[1-(2/8+3/8)]÷1/4
=(1-5/8)÷1/4
=3/8÷1/4
=3/2
(3)(0.875+1/4)(1/8-1/72)
=(7/8+2/8)(9/72-1/72)
=9/8 ×8/72
=1/8
(3)(7/5-2/5)÷4/5÷(3/7+3/2×4/9)
=1÷4/5÷(3/7+2/3)
=5/4÷(9/21+14/21)
=5/4÷23/21
=105/92=1又 13/92 （你 题目有可能打错了）
解方程
（1) 2X+0.4X=7.2
2.4X=7.2
X=7.2÷2.4
X=3
(2) 5-2/3X=1/3
2/3X=5-1/3
2/3X=14/3
X=14/3÷2/3
X=7
(3) 0.8+1/2X=0.9
1/2X=0.9-0.8
1/2X=0.1
X=0.2
(4)5X-3×7/10=7/5
5X=21/10+14/10
5X=35/10
X=7/10
以后自己做,这么小不自己算,你长大后会算什么?