Math basics › When you're stuck
I got a different answer from the book
Put your answer back in
Substitute it into the original problem. If it works, you may both be right — ½ and 0.5 and 2/4 are the same number.
Find the first line that is wrong, not the last
Go down your work until one line stops following from the one above it. That line is the mistake. Everything after it is just the mistake travelling.
Books are wrong sometimes
Rarely, but it happens. If you have checked twice and your answer survives substitution, say so and move on.