let's say "x" is the total amount of emails, and therefore, "x" is the 100%, and we know that 15 is the 20%, so
[tex]\bf \begin{array}{ccll} amount&\%\\ \cline{1-2} x&100\\ 15&20 \end{array}\implies \cfrac{x}{15}=\cfrac{100}{20}\implies \cfrac{x}{15}=5\implies x=75[/tex]
