Answer:
The speed of the combined mass after the collision is 2.1 m/s. 
Step-by-step explanation:
mass of runner, m = 70 kg 
speed of runner, u = 2.7 m/s 
mass of shortstop, m' = 85 kg 
speed of shortstop, u' = 1.6 m/s 
Let the velocity of combined system is v. 
Use conservation of momentum 
Momentum before collision = momentum after collision 
m u + m' u' = (m + m') v 
70 x 2.7 + 85 x 1.6 = (70 + 85) v
189 + 136 = 155 v 
v = 2.1 m/s