For margin of error to be a maximum of 3:
3 = z*(SD / sqrt(n)), where z is the z-score, SD is the standard deviation and n is the sample size.
z = 1.96 for a 95% confidence interval, and we are given SD = 15.
3 = 1.96*15/sqrt(n)
sqrt(n) = 9.8
n = 96.04 ~ 96 commercials.
That is impossible but I did some math so there are 2 ways that will equal more than 40.
6.7×6.0=40.2
6.5×6.2=40.3
Hope this helps!