39x5x4=39x20=39+39+39+39+39+39+39+39+39+39+39+39+39+3939+39+39+39+39+39=20+20+20+20+20+20+20+20+20+20+20+20+20+20+20+20+20+20+20+20+20+20+20+......=390x2=39x20=39x5x4
39*5*4=39*(5*4)=39*20=780
