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[latex] \left \{ {{mv=2mu} \atop { \frac{mv^2}{2} =\frac{2mu^2}{2}+Q}} \right. \\ \left \{ {{v=2u} \atop { \frac{mv^2}{2} =mu^2+Q}} \right. \\ \left \{ {{u= \frac{v}{2} } \atop { \frac{mv^2}{2} =\frac{mv^2}{4} +Q}} \right. \\ Q=\frac{mv^2}{4} \\ Q=\frac{4*5^2}{4}=25 [/latex] Дж