1/2*3+1/2*5
=1/2*(3+5)
=1/2*8
=4
3*(2/15+1/12)-2/5
=3*2/15+3*1/12-2/5
=2/5+1/4-2/5
=2/5-2/5+1/4
=1/4
1/3+3-(1/4+1/12)
=1/3+3-(3/12+1/12)
=1/3+3-1/3
=1/3-1/3+3
=3
3/4*5/7*4/3-1/2
=3/4*4/3*5/7-1/2
=5/7-1/2
=10/14-7/14
=3/14