(2/5 - 9/10) + 1/3 (3/14-11/28)-1/2 (2/7-1/14)-5/21

1) (2/5-9/10)+1/3=(4/10-9/10)+1/3=-5/10+1/3=-15/30+10/30=-5/30=-1/6
2) (3/14-11/28)-1/2=(6/28-11/28)-1/2=-5/28-1/2=-5/28-14/28=-19/28
3) (2/7-1/14)-5/21=(4/14-1/14)-5/21=3/14-5/21=9/42-10/42=-1/42