Giải thích các bước giải:
a) (-4+-2/3+7/4)*-12/13
=(-48/12 + -8/12 + 21/12)*-12/13
=(-35/12)*(-12/13)
=35/13
b)-1/2+5/6-3,12+5,1
=-3/6+5/6-3,12+5,1
=1/3-3,12+5,1
=50/150-468/150+765/150
=347/150
c) (4/15+7/12-9/20)*2,5+0,25
=(16/60 + 35/60 - 27/60)*5/2 +1/4
=2/5*5/2 +1/4
=4/4+1/4
=5/4
d) 30+2,8:(4/25-9/15-1/6)
=30 + 14/5:(24/150- 90/150 -25/150)
=30+14/5:(-91/150)
=30+(-60/13)
=330/13