How fast (in meters/second) is an object moving on the earth, if its speed is specified as Mach 0.2 ?

Answer 1

depends on the speed of sound, which is mainly dependent on temperature and density of air.

As a rule of thumb, 1225 km/h is Mach 1 in dry air, at 20C. But the actual speed of air varies with altitude (which affects temperature and density).

Mach .2 would be .2 times the speed of sound.