The given question is incorrect. The correct question is as follows.
If 20.0 g of  and 4.4 g of
 and 4.4 g of  are placed in a 5.00 L container at
  are placed in a 5.00 L container at  , what is  the pressure of this mixture of gases?
, what is  the pressure of this mixture of gases?
Explanation:
As we know that number of moles equal to the mass of substance divided by its molar mass.
Mathematically,   No. of moles = 
Hence, we will calculate the moles of oxygen as follows.
        No. of moles = 
      Moles of  =
 =  
 
                             = 0.625 moles 
Now,   moles of 
                                       = 0.1 moles
Therefore, total number of moles present are as follows.
Total moles = moles of  + moles of
 + moles of 
                     = 0.625 + 0.1 
                     = 0.725 moles
And, total temperature  will be:
                     T = (21 + 273) K = 294 K
According to ideal gas equation,  
                          PV = nRT
Now, putting the given values into the above formula as follows.
                 P =  
 
                    = 
                     =  atm
 atm
                     = 3.498 atm 
or,                = 3.50 atm (approx)
Therefore, we can conclude that the pressure of this mixture of gases is 3.50 atm.