Fraction of molecules are plotted against velocity of molecules, according to Maxwell-Boltzmann distribution law. The velocity corresponding to the points $\mathrm{X}, \mathrm{Y}$ and $\mathrm{Z}$ are:

Here,

 ${\mathrm{v}}_{\mathrm{mp}}=$ most probable velocity.
${\mathrm{v}}_{\mathrm{av}}=$ average velocity.
$v_{\mathrm{rms}}=$ root mean square velocity.