Let n(X_4) = n for brevity.
Then n(X_1) = n(X_2) = n(X_3) = 6n,
n(X_4) = n(X_5) = n(X_6) = n.
Hence, you have an equation
3n + 3*(6n) = 525, or
21n = 525.
It implies n = = 25.
Then n(X_1) = 6n = 150.
Answer. n(X_1) = 150.