Jawabanmu

2014-08-28T19:35:17+07:00
A. 2/5 - 1/2 + 3/10
    4/10 - 5/10 + 3/10
    -1/10 + 3/10 = 2/10 atau 1/5
b. 3/7 + 4/21 - 3/14
    12/28 + 8/28 - 6/28
    20/28 - 6/28 = 14/28 atau 1/2
2014-08-28T19:38:27+07:00
2/5 - 1/2 +3/10 =disamakan bawahnya = 4/10 - 5/10+ 3/10 = 2/10
3/7 + 4/21 - 13/14 = 18/42 + 8/42 - 39/42 = -13/42 

 Maaf 
kalau ada keliru jawabanya