Suppose there were 1000 potential customers, then (on avaerge) 20 will have seen

the magazine add, and 200 will have seen the TV add and 10 will have seen

both. So 20+200-10=210 will have seen at least one of the adds, and 790

will have seen neither.

So our sales are 210/3 + 790/10 = 149 out of 1000 potential customers.

Hence th probability that a random customer buys is 149/1000 = 0.149

or 14.9%.

RonL