Answer : 20.
Consider two teams A and B.
For simplicity, I'm assuming A is winning playoff.
So, the number of ways for winning A is:
-Winning first 3 games $\rightarrow$ 1 way
-Winning at the end of 4th game $\rightarrow$ 3C2=3 ways
-Winning at the end of 5th game $\rightarrow$ 4C2=6 ways
Total number of matches in which A can win = 1+3+6 = 10 ways
Similarly, total number of matches in which B can win = 10
$\therefore$ different ways in which playoff can occur = 10 + 10 = 20