During a sale at a bookstore, joseph buys a book at full price. he is given a 50 percent discount on a second book of equal or less value. if joseph buys two books with a full price of $15 and $10 respectively, by what percent is the total cost of the two books reduced during the sale?

Joseph bought a book at full price for $15, then on the sale he got the second one for half price, but we know the second one was $10, and we know that's half the regular price, so the regular price must be twice as much of that, namely $20.

Joseph ended up paying only $25 for both, including the discount on the second, however if there wasn't any sale, he would have paid full price for both, namely 15 + 20, or 35.

so, he didn't pay $35, he paid $25, he saved 10 bucks.

if we take 35 to be the 100%, what is 10 off of it in percentage?

The answer is biodiversity resulting from new ancestors.