a+b=20
b+c=30
3a+7b+4c=3a+3b+4b+4c=3(a+b)+4(b+c)=3*20+4*30=60+120=180
a+b=33
a+c=11
5a+3b+2c=3a+3b+2a+2c=3(a+b)+2(a+c)=3*33+2*11=99+22=121
