In a mixture of oxygen and nitrogen gas, 80.0 percent of the total gas pressure is exerted by the nitrogen. if the total pressure is 2.0 atm, what pressure does the oxygen exert?

0.40 atm

How did we get 0.40?
Use the formula of Dalton's Law of partial pressure. 
Work it out by substituting the given values.
Given:
Total gas exerted by nitrogen- 80%
Total pressure- 2.0 atm
pressure of oxygen-?
P denotes total pressure exerted by nitrogen and n denotes total pressure of the mixture.
Poxygen = P/n
             = 0.8 /2.0
             = 0.4

(80 % is changed into decimal number, 0.8)

They are all decomposition reactions.