Answer:
6 tables.
Step-by-step explanation:
If he has 15 shelves, we can put the overall number of books off to the side for a moment. First calculate how many books he COULD put onto shelves. This would be 7*15, since each of fifteen shelves can hold 7 books. Counting by fifteens or using a calculator allows us to see that the shelves can hold 105 books.
Going back to the original number of books, we subtract 105 from the original value. The equation at this point is 231 - 105. The result is 126 books left to put on top of tables. But we're not done yet!
Since a table can hold 25 books, we need to divide the remaining number of books by 25. That would be 126/25. Doing this gives us 5 tables we would need, plus one book let over. Since we've run out of shelves, we MUST use another table just for the final book. That's 5+1, or 6 tables. Let's not forget to label our answer.
