a) a+n+r=120
a=18+n
r=(a+n)/3=(18+n+n)/3=(18+2n)/3
18+n+n+(18+2n)/3=120 (18+2n)+(18+2n)/3=120 3*(18+2n)+(18+2n)=3*120
4*(18+2n)=360 72+8n=360 8n=288 n=36 (36 bile negre)
a=18+36=54 (54 bile albe)
r=(18+2*36)/3=30 (30 bile rosii)
b) probabilitatea de a extrage o bila rosie=nr. bile rosii / nr. total bile=30/120=1/4