Notice that:
(1)
[tex]\frac{8}{20}=\frac{2\cdot4}{5\cdot4}=\frac{2}{5}\text{.}[/tex]Therefore:
[tex]\frac{8}{20}=\frac{2}{5}\text{.}[/tex](2)
[tex]\begin{gathered} \frac{12}{28}=\frac{3\cdot4}{7\cdot4}=\frac{3}{7}, \\ \frac{21}{49}=\frac{3\cdot7}{7\cdot7}=\frac{3}{7}\text{.} \end{gathered}[/tex]Therefore:
[tex]\frac{12}{28}=\frac{21}{49}\text{.}[/tex](3)
[tex]\begin{gathered} \frac{10}{18}=\frac{5\cdot2}{9\cdot2}=\frac{5}{9}, \\ \frac{15}{27}=\frac{5\cdot3}{9\cdot3}=\frac{5}{9}\text{.} \end{gathered}[/tex]Therefore:
[tex]\frac{10}{18}=\frac{15}{27}\text{.}[/tex]Answer: The 3 sets have in common that in each case both fractions represent the same number.
