Sorry for such a simple question. Unfortunately, this problem was originally written in Japanese and was just translated in English by a coworker. I don't know if there was a problem during the translation but we cannot produce the same answer as was shown in the Japanese answer key. Problem: A spring with a spring constant of K and with negligible mass was suspended, and vibrated with a small plate attached to its lower end. Then a cycle of To was obtained. When a weight with a mass of m1 was placed on the plate, the spring lowered by a length of a. Next, after the weight with a mass of m1 was removed, the spring was vibrated with another substance placed on the plate. In this case, if the obtained cycle was T, what is the mass m of this substance?