Solution:
Let the number of wickets taken by Zahir be x.
=> Number of wickets taken by Harbhajan = 2x - 3.
According to the given condition,
x(2x – 3) = 20
=> 2x2 – 3x = 20
=> 2x2 – 3x – 20 = 0
Therefore, the required quadratic equation is 2x2 – 3x – 20 = 0.