It takes 1 person working 10 hours to figure out the solution to a math problem.  If you add a second person who's equally good at math, how long will the two of them have to work to solve the same problem?
Since you're doubling the number of people working on the task, that means you will be able to reduce the amount of time worked in half if they're of equal skill.
Therefore, you take the original time of 10 hours and perform 10/2= <<10/2=5>>5 hours spent working on the problem.
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