Quizzes & Puzzles2 mins ago
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(Thanks to Cetti for showing me the tinurl.com site.)
Presumably razorback knows that the answer is 25, he (or she) does not understand why this is so. The previous answers explain why it has to be so, rather like why you have to not allow division by zero. If razorback would explain his(or her) difficulty a bit better, others could attempt a better answer.