1.(9/14-5/21):2,5/6=(9/14•3)-(5/21•2):2,5/6•8=(27/42-10/42):2,40/42=1,10/40
2.(180:240)•100%=75%
3. -3(b+4)-5(2-3b)=-3b-12-10+15b=12b-22
с * 5 + 2,859 = 3,454
с*5=3,454-2,859
с*5 = 0,595
с= 0,595 : 5
с=0,119
18,512 - а*3 = 17,732
а*3 = 18,512 - 17,732
а*3 = 0,78
а= 0,78 : 3
а=0,26