(x-0.5)*2+x=420
2x-1+x=420
3x=421
x=140
(140-0.5)*2=139.5*2=238
2/3+2/9+2/27+... b1=2/3 q=1/3<1 S=b1/1-q=1/3:(1-1/3)=1/3:2/3=1/2