Suppose that Mathilde had $m$ coins at the start of last month and Salah had $s$ coins at the start of last month. From the given information, 100 is 25% more than $m$, so $100=1.25m$ which means that $m=\frac{100}{1.25}=80$. From the given information, 100 is 20% less than $s$, so $100=0.80s$ which means that $s=\frac{100}{0.80}=125$. Therefore, at the beginning of last month, they had a total of $m+s=80+125=205$ coins.