Snowplow Math Puzzle

One morning it starts to snow at a constant rate. Later, at 6:00am, a snowplow sets out to clear a straight street. The plow can remove a fixed volume of snow per unit time, in other words its speed it inversely proportional to the depth of the snow. If the plow covered 10 miles from 6 to 7am and 5 miles from 7 to 8am, what time did it start snowing?

Links

• MindYourDecisions asks the same type of problem in this YouTube video.
• Discussion about this problem in my forum at Wizard of Vegas.
• mathproblems.info, where this problem and many more are asked and solved.