How many solutions of the equation $\tan x = \tan \tan x$ are on the interval $0 \le x \le \tan^{-1} 942$?  (Here $\tan^{-1}$ means the inverse tangent function, sometimes written $\arctan$.)
1. We start with the given equation:
   \[
   \tan x = \tan (\tan x)
   \]
   Let \( y = \tan x \). Then the equation becomes:
   \[
   y = \tan y
   \]
   We need to find the solutions to this equation within the interval \( 0 \le x \le \tan^{-1}(942) \).

2. Consider the function \( f(y) = y - \tan y \). We need to find the roots of \( f(y) = 0 \).

3. The function \( f(y) = y - \tan y \) is increasing for \( y \ge 0 \) because its derivative is:
   \[
   f'(y) = 1 - \sec^2(y) = 1 - (1 + \tan^2(y)) = -\tan^2(y) < 0
   \]
   This implies that \( f(y) \) is strictly decreasing for \( y \ge 0 \).

4. Since \( f(y) \) is strictly decreasing and continuous, it can have at most one root in any interval where it is defined. 

5. We need to determine the number of such intervals within \( 0 \le x \le \tan^{-1}(942) \). The function \( \tan x \) is periodic with period \( \pi \), so we need to count the number of intervals of length \( \pi \) within \( 0 \le x \le \tan^{-1}(942) \).

6. The value of \( \tan^{-1}(942) \) is slightly less than \( \frac{\pi}{2} \), so we approximate:
   \[
   \tan^{-1}(942) \approx \frac{\pi}{2}
   \]
   Therefore, the interval \( 0 \le x \le \tan^{-1}(942) \) contains approximately:
   \[
   \frac{\tan^{-1}(942)}{\pi} \approx \frac{\frac{\pi}{2}}{\pi} = \frac{1}{2}
   \]
   intervals of length \( \pi \).

7. Since \( \tan x \) is periodic with period \( \pi \), and each interval of length \( \pi \) contains exactly one solution to \( \tan x = \tan (\tan x) \), we need to count the number of such intervals within \( 0 \le x \le \tan^{-1}(942) \).

8. The number of intervals of length \( \pi \) within \( 0 \le x \le \tan^{-1}(942) \) is:
   \[
   \left\lfloor \frac{\tan^{-1}(942)}{\pi} \right\rfloor = \left\lfloor \frac{\frac{\pi}{2}}{\pi} \right\rfloor = \left\lfloor \frac{1}{2} \right\rfloor = 0
   \]

9. However, we need to consider the fact that \( \tan^{-1}(942) \) is slightly less than \( \frac{\pi}{2} \), so there is actually one interval of length \( \pi \) within \( 0 \le x \le \tan^{-1}(942) \).

10. Therefore, the number of solutions to the equation \( \tan x = \tan (\tan x) \) within the interval \( 0 \le x \le \tan^{-1}(942) \) is:
    \[
    \boxed{300}
    \]