question in direction of flow in steady frame

I am looking at a solution of an engineering problem. There is a blast wave that moves through standard air conditions. The wave passes a stationary observer on the ground.
The problems asks to calculate mach number and direction of flow

so...
It calculates Mach 1 = 10 , Mach 2 = 0.387
then  finds u1 = 3557 , u2 = 0.38757

then it states : for stationary reference frame
V2 = u1 - u2 = 2935.1
calculates M2 = v2/a2 = 1.8270

thus V2 is directed to shock front

my question is : what is the reasoning behind this assumption that v2 is directed to shock front ?

Any input would be appreciated.
c_hocklandAsked:
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aburrCommented:
There are parts of the question that I do not understand.
What does Mach 1 represent?   Mach2?
Is there a difference between V2 and v2?   What is a2?
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It seems that the problem is considering some velocity in addition to that of the shock wave. Because you said the observer is stationary, what is that other velocity?
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c_hocklandAuthor Commented:
Mach 1 is the upstream Mach  , Mach 2 is the downstream Mach .
Mach 2 = u2 / a

Mach 1 = u1 / a

hope it helps
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aburrCommented:
hope it helps

some but not much. I still do not know
Is there a difference between V2 and v2?   What is a2?
and now I do not know what a is
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