See the title/poll.
I swear to god, my employer (a for-profit test-prep company) can’t makes up its mind–we’ve bounced back and forth between formats four times in four editions.
Just for preference, all at the end of the book.
People usually expect them to be at the back.
There is something psychological at work with me that if they are in the chapters, I will cheat and peek. If they are in the back, I won’t. So I prefer them in the back.
I prefer them at the end of the chapter. I find it much easier to check my answers when I just have to flip a page or two instead of flipping half the weight of the book just to find the damn things. If you insist on putting them at the back, can you at least give some decent division between sections instead of just running from one to another?
How about quiz answers on a separate tear-out page, right after each quiz, ready to just tear out and hand in? 
ETA: Benefit for the publisher: Totally destroys the re-sale value of the book too!
I sort of prefer the end of the chapters but it’s not a strong preference.
I don’t know about anyone else’s experience but in mine, the problems to be handed in are the ones without answers. If I get stuck on one of those, I’ll try to work one with an answer to see if I am going in the right direction. I always thought that was the point of having the answers to only half the problems.
Why would they be anywhere except the back of the book?
Other: The answers should be given to an old hermit who lives high up on a mountain.
In Spain another frequent option is a so-called separata, a sort of teacher’s booklet.
Electronic textbook with the answer(or clue!) coming up after you attempt to answer.
Otherwise, the back.
This thread reminds me of teacher’s editions. The good ones are wraparounds- all answers, notes, etc. right on the page in the extra big margins.
At the end, with different colored pages (or at least the border of the pages).