a) [(16-4+8):a]= 8-3
20:a= 5
a=20:5
a=4
b) 5+15x(25-75:5+25:a)=1150:5
5+15x(25-15+25:a)=230
5+15x25-15x15+15x (25:a)=230
5+375-225+15x (25:a)=230
155+15x (25:a)= 230
15x (25:a)=230-155
25:a=75:15
a=25:5
a=5
c) 3+2x { [ 72-12x(a-3)]-9x4}=27
3+2x [(72-12x a +36)-36]=27
3+2x(72-12x a +36-36)=27
3+2x(72-12x a)=27
3+2x72-24x a =27
3+144-24x a =27
-24x a= 27-3-144
24x a=120
a=120:24
a=5