a) (1/2+2/3-1/4) - (2/5+1/4-1/2)=6+8-3/12 - 8+5-10/20= 11/12-3/20= 55-9/90=46/90=23/45
b)10*[1/2+(7/12-1/8):11/6]= 10*(1/2+ 14-3/24 :11/6)=10*(1/2+11/24:11/6)= 10*( 1/2+ 11/24 *6/11)=
10* (1/2+ 1/4)= 10* 2+1/4=10*3/4=15/2
c) 2+10*(4/5+1/2)+1/5:(3/5-1/10)= 2+10*8+5/10 + 1/5: 6-1/10= 2+10* 13/10+ 1/5: 5/10= 2+13+1/5*10/5= 15+2/5=77/5