a) [680:20+4x(2+12:12)]X100=
=[34+4x(2+1)]X100=
=(34+12)X100=
=46X100= 4600
b) {[(2+3x2)x2-4]:3+5}x2-5=
={[(2+6)x2-4]:3+5}x2-5=
=[(16-4):3+5]x2-5=
=(12:3+5)x2-5=
=(4+5)x2-5=
=18-5=13
c) 9+10x{9+10x[9+10x(9+10x9)-9]-9}-9=
=9+10x{9+10x[10x(9+90)]-9}-9=
=10x10x990=
=99000