Answer: He can buy 8 DVD's.
Explanation:
Since, The number N of DVDs Phil can buy is inversely proportional to the cost C of a DVD.
Thus,

⇒
------- (1)
Where k is the proportional constant.
And, According to the question,
If the cost of one DVD is $16, then he can buy 10 DVD,
That is, If C = $16 then N = 10
Put these value in equation (1),

⇒ k = 160
Thus, the relation between N( number of DVD's ) and C( Cost of DVD's ) is, ( By putting the value of k in equation (1) )
If C = $20,
⇒
= 8
Therefore, If the cost of one DVD is $20, he can buy 8 DVD's.