In a probability model, the sum of all probabilities must equal 1. Given the probabilities for the letters Y, C, A, R, and E, we can find the value of \( x \) by setting up the following equation:
\[ P(Y) + P(C) + P(A) + P(R) + P(E) + P(S) = 1 \]
Substituting the values we have:
\[ 0.25 + 0.18 + 0.30 + 0.17 + x = 1 \]
Now, let's calculate the sum of the known probabilities:
\[ 0.25 + 0.18 + 0.30 + 0.17 = 0.90 \]
Now we can plug this back into the equation:
\[ 0.90 + x = 1 \]
To find \( x \), we subtract 0.90 from both sides:
\[ x = 1 - 0.90 = 0.10 \]
Therefore, the value of \( x \) is:
\[ \boxed{0.10} \]