If salary average of 4 players is making $200,000/yr, what's the total payroll? It should be $200K x 4 or $800,000.
Now if you add in that 5th player at $400,000 what is the total payroll of all 5 players? It should be $800,000 + $400,000 = 1,200,000. From there you can find the average of the 5 players by dividing!