How to change a continuous signal to form a discrete signal?
When your continuous-time signal $$x(t)= 2\cos(200\pi t)+8\cos(250\pi t)$$ is sampled at a rate of $F_s = 400$ Hz, (or sampled with a period of $T_s= \frac{1}{F_s} = \frac{1}{400}$ seconds per sample), you can use the following to find the mathematical representation of the discrete-time signal $x[n]$ :
$$ x[n] = x_c(t)|_{t=nT_s} $$
Applying this to $x(t)$ leads to:
$$x[n] = x(nT_s)= 2\cos(200\pi n T_s)+8\cos(250\pi n T_s)$$
$$x[n] = x(n/400)= 2\cos(200 \pi n /400)+8\cos(250\pi n /400)$$
$$x[n] = 2 \cos(\frac {2\pi}{4} n) + 8 \cos(\frac {5\pi}{8} n)$$
Now observing the pattern of sinusoidal signals in discrete time as $\cos(\omega_0 n)$ where $\omega_0$ is the discrete time radian frequency between $-\pi$ and $\pi$ (or $0$ to $2\pi$ if you prefer so) you can deduce your discrete time frequencies as $\pi/2$ and $5\pi/8$
Note that $\omega_0 = \pi$ will be the highest frequency discrete time sinusoid.