Let \( x \) be the number of patients admitted to hospital A, and \( x + 21 \) be the number of patients admitted to hospital B.
Given information:
Average recovery days for hospital A = \( \frac{200}{x} \)
Average recovery days for hospital B = \( \frac{152}{x + 21} \)
The problem states that the average for hospital A exceeds the average for hospital B by 3 days. This yields the equation:
\[ \frac{200}{x} = \frac{152}{x + 21} + 3 \]
Isolate the terms with \( x \):
\[ \frac{200}{x} - \frac{152}{x + 21} = 3 \]
To eliminate the denominators, multiply both sides by \( x(x + 21) \):
\[ 200(x + 21) - 152x = 3x(x + 21) \]
Expand both sides of the equation:
\[ 200x + 4200 - 152x = 3x^2 + 63x \]
Combine like terms:
\[ 48x + 4200 = 3x^2 + 63x \]
Rearrange into a standard quadratic form \( ax^2 + bx + c = 0 \):
\[ 3x^2 + 15x - 4200 = 0 \]
Apply the quadratic formula: \( x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \). Here, \( a = 3 \), \( b = 15 \), and \( c = -4200 \).
Substitute the values:
\[ x = \frac{-15 \pm \sqrt{15^2 - 4(3)(-4200)}}{2(3)} \]
Calculate the discriminant:
\[ x = \frac{-15 \pm \sqrt{225 + 50400}}{6} \]
\[ x = \frac{-15 \pm \sqrt{50625}}{6} \]
Calculate the square root:
\[ x = \frac{-15 \pm 225}{6} \]
Determine the possible values for \( x \):
The number of patients admitted to hospital A is \( \boxed{35} \).