Students’ and adults’ knowledge of prep (Campbell & Xue, 2001, LeFevre, & Liu, 1997, LeFevre et al., 1996, Metcalfe & Campbell, 2008) has been studied extensively. However, a few studies have analyzed students’ strategies in mental calculations of multi digit multiplications (Baek Jae-Meen, 1998; Heirdsfield, A. et al. 1999; Murray, H., et al. 1994; Lemonidis, 2013). In addition, there are no studies analyzing adults’ strategies or examining their mental flexibility in two-digit multiplications, as well as comparing this flexibility with their knowledge of multiplication tables. The main research questions in this paper are (a) which strategies are used by prospective teachers, (b) how flexible they are in mental calculations of two-digit multiplications and (c) how this flexibility is associated with their knowledge of the multiplication tables. Their flexibility in strategies employed to multiply two-digit numbers was examined by using the method choice / no-choice by Lemaire & Siegler, (1995). The results showed that prospective teachers are not flexible in two-digit multiplications and they mostly use the written algorithms mentally in order to calculate. Moreover, their flexibility to mentally calculate two-digit multiplications is associated with their knowledge in prep and their prep response time.