Can you please show me step by step how to solve this thanks so much

In a gas mixture of 35% He and 65% O2 the total pressure is 800 mm Hg. What is the partial pressure of O2?

A) 35 mm Hg

B) 65 mm Hg

C) 100 mm Hg

D) 280 mm Hg

E) 520 mm Hg