Observed correlation of [Zn/Mg] with [Mg/H] for the Galaxy obtained from the SAGA database (Suda et al. 2008). The [Zn/Mg] ratio decreases with increasing [Mg/H] and upturns around [Mg/H] = −1. These two discrete trends are attributed to a decrease in the event frequency of MR-SNe as a function of the metallicity and an accumulation of the contribution to Zn from SNe Ia, respectively. As a result, the observed [Zn/Mg] ratio reflects the ratio of contributions from different types of SNe for each element with elapsed time: i.e., MR-SNe to CCSNe for [Mg/H] ≲ −1 and MR-SNe plus SNe Ia to CCSNe for [Mg/H] ≳ −1.