Answer:
978mmHg
Step-by-step explanation:
The total pressure of a gas mixture is defined as the sum of the pressure of each gas in the mixture.
If the mixture of gases you have 200mmHg for O₂, 350mmHg for N₂ and 428mmHg for He, total pressure of the mixture is:
200mmHg + 350mmHg + 428mmHg = 978mmHg