How would you say a mach speed of 0.64?

To articulate a Mach speed of 0.64 correctly, saying "mach point six four" accurately conveys the intended meaning while adhering to standard aviation communication. In this context, "point" is commonly used to denote the decimal, making it clear that you are referencing a fractional Mach number. Saying "six" and "four" individually helps eliminate confusion, ensuring that the speed is clearly understood by all parties involved.

Using the term "point" before the digits is standard practice in aviation and helps distinguish numerical values clearly, especially since precise communication is crucial in air traffic control. The other phrasing options, although they use numbers correctly, may not fit the conventional ways of expressing decimal figures in Mach speed.