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Supersonic plane fly with supersonic speed.
Supersonic speed is the speed which is greater than the speed of sound(approximately 340 m/s). It is measured in mach numbers.

Mach\ number= \frac{speed\ of\ air-craft}{speed\ of\ sound}
1 5 1
Supersonic plane fly with the speed that exceeds the speed of sound.

Such speeds are  represented by Mach no.