Let the amount of concentrate A in mixture P be \( x \) ml, and the amount of concentrate B be 40 ml. For mixture Q, 90 ml of concentrate A is used, and the amount of concentrate B is \( x \) ml.
The concentrations in both mixtures are the same. Therefore, the ratio of concentrate A to concentrate B in each mixture must be equal. We can set up the following equation:
For mixture P:
\[
\frac{x}{40} \quad \text{(ratio of A to B in P)}
\]
For mixture Q:
\[
\frac{90}{x} \quad \text{(ratio of A to B in Q)}
\]
Since the ratios are equal, we have:
\[
\frac{x}{40} = \frac{90}{x}
\]
Multiplying both sides by \( x \times 40 \), we get:
\[
x^2 = 3600
\]
Solving for \( x \), we find:
\[
x = 60
\]
Thus, the value of \( x \) is 60 ml.
Final Answer:
\[
\boxed{\text{The two quantities are equal.}}
\]