1/3+2/7-1/4
=28/84+24/84-21/84
=31/84
4/5-(2/15+1/9)
=12/15-2/15-1/9
=10/15-1/9
=6/9-1/9
=5/9
3/4-(1/2-2/7)
=3/4-2/4+2/7
=1/4+2/7
=7/28+8/28
=15/28
1/3+2/7-1/4
=28/84+24/84-21/84
=31/84
4/5-(2/15+1/9)
=12/15-2/15-1/9
=10/15-1/9
=6/9-1/9
=5/9
3/4-(1/2-2/7)
=3/4-2/4+2/7
=1/4+2/7
=7/28+8/28
=15/28