1. **State the problem:** Factorise the expression $5X^2 - 25XY$. 2. **Identify common factors:** Both terms have a common factor of $5X$. 3. **Apply the distributive property:** Factor out $5X$ from each term: $$5X^2 - 25XY = 5X(X) - 5X(5Y)$$ 4. **Write the factorised form:** $$5X(X - 5Y)$$ 5. **Explanation:** We took out the greatest common factor $5X$ to simplify the expression into a product of $5X$ and the binomial $(X - 5Y)$. **Final answer:** $5X(X - 5Y)$