Thanks, neat question.
I'm sure this may not work every time, but I used the process of elimination based on the sums provided as answers.
I first assumed that there was no error, and that @first would be 50000 (truncated due to varchar(5)).
That led me to get @number + @first + @second = 500000 + 50000 + 50000 = 600000.
600000 wasn't one of the sums given as an answer, so I figured I should pick the error as my answer. Had 600000 been an available answer choice, I probably would have picked it and gotten the answer wrong.
Just my two cents if you want to make it tougher in the future to guess at the answers.