Let us determine the distance from the point of the beginning of the motion to the point of first impact to the ground. This distance can be calculated (see https://courses.lumenlearning.com/boundless-physics/chapter/projectile-motion/) as
"D_1 = \\dfrac{v_0^2\\sin2\\theta}{g} = \\dfrac{20^2\\sin90^\\circ}{g} = 40\\,\\mathrm{m}."
Coefficient of restitution (see https://en.wikipedia.org/wiki/Coefficient_of_restitution) is defined as the ratio of the velocity after impact and initial velocity. The distance is proportional to "v_0^2" , so "D_1 = 0.5^2\\cdot D_1 = 10\\,\\mathrm{m}" . , "D_3 = 0.5^2\\cdot D_2 = 2.5\\,\\mathrm{m}," and so on. The sum will be "\\approx 53.3\\,\\mathrm{m}."
But if we define the coefficient of restitution as the ratio of kinetic energy, we get the distances to decrease as "0.5^1" , so "D_2 = 0.5D_1 = 20\\,\\mathrm{m}," "D_3 = 0.5D_2 = 10\\,\\mathrm{m}, D_4 = 0.5D_3 = 5\\,\\mathrm{m}," and so on. The sum will be "40+20+10+5+2.5+1.25 + \\ldots \\approx 80\\,\\mathrm{m}."
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