ex 1:
A=28/105-8/5
=4/15-8/5
=4/15-8×3/5×3
=4/15-24/15
=-20/15=-4/3
B=1/4+-5/24
=1×24/4×24+-5×4/24×4
=24/96+-20/96
=4/96=1/24
C=4/-3+7/15
=4×5/-3×5+7/15
=20/-15+7/15
=-13/15
D=(8/15-7×3/5×3)-(-1×3/6×3+2×2/9×2)
=(8/15-21/15)-(-3/18+4/18)
=-13/15+1/18
=-13×18/15×18+1×15/18×15
=234/270+15/270
=249/270=83/90
ex 2:
A=-
B=×
C=×
D=-
