Let the velocity with which ball strikes the floor & rebounds be v and v' respectively.Using energy conservation: mgh=\\dfrac { 1 }{ 2 } m{ v }^{ 2 }\\quad \\quad -(1)\\\\ mg(0.64h)=\\dfrac { 1 }{ 2 } m{ v' }^{ 2 }\\quad \\quad -(2) Using (1) & (2) \\dfrac { 0.64 }{ 1 } =\\dfrac { { v' }^{ 2 } }{ { v }^{ 2 } } \\\\ \\Rightarrow \\dfrac { { v' } }{ { v } } =0.8 Therefore coefficient of restitution is 0.8