Explanation: \(\text{Hints: } H = \frac{v^2}{2g}, \, mgx = mg(h - x)\) \(\text{Solve: } H = \frac{v^2}{2g} = \frac{400}{2 \times 9.8} = 20.4\) \(\therefore h = 10 + H = 10 + 20.4 = 30.4 \approx 30\) \(\text{শর্তমতে, } mgx = mg(h - x)\) \(h = \text{মোট উচ্চতা}; \, x = \text{ভূমি হতে যে উচ্চতায় বিভব শক্তি ও গতিশক্তি সমান}\) \(\implies x = h - x \implies 2x = 30 \implies x = 15\) \(\text{Ans. (C)}\)