# Inlet Mach No for a centrifugal pump?

Recently I have come across a question as follows:

At the eye tip of a centrifugal impeller, blade velocity is 200 m/s while the uniform axial velocity at the inlet is 150 m/s. If the sonic velocity is 300 m/s, then the inlet Mach number of flow will be?

My answer is $$\frac{150}{300} = .5$$,

but the correct answer is $$0.83$$, with an explanation:

$$M = \frac{\sqrt{(blade\ velocity)^2 + (axial\ velocity)^2}}{sonic velocity} = 0.83$$

My question is: when we are considering the inlet mach why are we considering the blade velocity also why not just inlet flow velocity?