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Where do you get 3, 13 and 24 from? Did you read the terms and conditions? You're supposed to find the answer by simply replacing the plus signs with either a plus or minus or divide or multiply and perhaps adding brackets to make sense of the precedence!

I missed that part, I thought It was just a pattern finding problem

That's okay! I wish you'd left the comment unedited for others to see. I still haven't figured out how I should treat those who leave two comments/answers. I have to think up of something and add it to the rules section! :]