Answer:
m = 5 [mg]
Step-by-step explanation:
We must remember that the definition of linear momemtum is defined as the product of mass by distance.
P = m*v
P = momentum = 40 [mg*m/s]
m = mass [mg]
v = velocity = 8 [m/s]
Now clearing m:
m = P/v
m = 40/8
m = 5 [mg]