On most social media platforms, 64% of all ads are targeted ads. Ad-blockers attempt to block such targeted ads before they are delivered to the user. One such ad-blocker has a 42% chance of blocking an ad if it is actually targeted, and a 19% chance of blocking an ad that is not targeted. a. If an ad is blocked by the ad-blocker, what is the probability that it was not a targeted ad? b. If the user is served an ad which was not blocked, what is the probability that it was targeted?