To determine the sequence pattern, let’s examine the changes between consecutive terms:

20 → 10: Multiply by $0.5$. 10 → 10: Multiply by $1$. 10 → 15: Multiply by $1.5$. 15 → 30: Multiply by $2$. 30 → 75: Multiply by $2.5$.

Following this progression, the next operation should be multiplying by 3: $75 \times 3 = 225$

Thus, $X = 225$.