<human>:  Erin is sorting through the library books to decide which ones to replace. She finds 8 less than 6 times as many obsolete books as damaged books. If she removes 69 books total, how many books were damaged?
<bot>: Define a variable ** Let o be the number of obsolete books and d be the number of damaged books. We know that o + d = 69 and o = 6d - 8.
Write an equation ** Substituting the first equation into the second equation, we get 6d - 8 + d = 69
Combine like terms ** Combining like terms, we get 7d - 8 = 69
Add 8 to both sides ** Adding 8 to both sides, we get 7d = 77
Divide by 7 ** Dividing both sides by 7, we get d = 11
#### 11