4. A search engine company has recorded the following information about the effectiveness of their advertising for a certain product: the chance of user clicking on the product's ad appearing on the side banner during each search is $1 / 50$, the chance of user clicking on the product's ad site appearing within the search results is $1 / 5$, while the chance that they click on both is 1/100. Of the users that clicked on the product's ad site, $20 %$ then made a purchase, and of the users that did not click on the product's ad site only $2 \ %$ made a purchase. Calculate the chance of a user making a purchase. SP.AS. 1580