Cross brand comparison

Hi, 

@4:30, the instructor takes value for brand 5 as 2.40$ and mentions 'moreover the cost of brand 4 is 2$'. Why are they comparing the elasticities at 2$ and 2.40$?

Also from that point onwards, I don't understand the inferences that the instructor makes. Please help! Why would brand 5 decreasing price be beneficial? Why net elasticity is 0.5? Why X * 2 = 1.5%? Help!