Good question. It is an abbreviated way of talking which is conventional. A grammatically standard version would be: "10, when divided by 5, is 2."

Sorry if I've misunderstood, but I'll try to answer. If it's a maths problem, then the full question is 'what is 10 divided by 2?'. You can also form other sentences like: - the answer is 10 divided by 2 - the calculation (needed) is 10 divided by 2 In maths, we tend to shorten sentences to the bare minimum. I guess this helps focus on the maths and not confuse people with English; we're not that strict about grammar! Hope this helps.