How fast is mach three?
The speed of sound, or Mach number, varies with temperature, air density and altitude. At sea level and 68 degrees Fahrenheit, Mach 1 is about 761 miles per hour, so Mach 3 is 2283 miles per hour. At 40,000 feet and -70 degrees Fahrenheit, Mach 1 and Mach 3 are 660 and 1980 m.p.h. respectively.
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