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