A mass of \(20 \mathrm{~kg}\) slides back and forth on a dry surface due to the action of a spring having a stiffness of \(10 \mathrm{~N} / \mathrm{mm}\). After four complete cycles, the amplitude has been found to be \(100 \mathrm{~mm}\). What is the average coefficient of friction between the two surfaces if the original amplitude was \(150 \mathrm{~mm}\) ? How much time has elapsed during the four cycles?