[maths]x_A=Ut\\y_A=-\frac{g}{2}t^2+4h\\x_B=V(t-10)\\y_B=-\frac{g}{2}(t-10)^2+h\\when\,\,y_A=0,T_A=\sqrt{\frac{8h}{g}}\\when\,\,y_B=0,T_B=10+\sqrt{\frac{2h}{g}}\\T_A=T_B\\\sqrt{\frac{8h}{g}}=10+\sqrt{\frac{2h}{g}}\\\sqrt{\frac{2h}{g}}\left ( 2-1 \right )=10\\\frac{2h}{g}=100\\h=50g\\\\Also,\,x_A=x_B\,at\,T_A\\U\sqrt{\frac{8h}{g}}=V\left ( \sqrt{\frac{8h}{g}}-10 \right )\\U=V\left ( 1-\frac{10}{\sqrt{\frac{8h}{g}}} \right )\\U=V\left ( 1-\frac{10}{20} \right )\\\therefore V=2U[/maths]
