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How fast must you drive?

You have a thirty mile commute to work that you normally drive at sixty mph.  If you start out ten minutes late, how fast do you have to drive to get to work on time?

Ans: 90 mph

Normal drive time is one half hour.  To make it in twenty minutes (⅓ hr)  requires a speed of 30 / (⅓) = 90 mph.