Given the ratio;
[tex]15\colon21[/tex]To simplify the above ratio, we would change the ratio to a fraction then divide the numerator and the denominator by the same value, and till we get to the lowest possible term.
This gives,
[tex]\begin{gathered} \frac{15}{21}\mleft\lbrace\text{Divide numerator and denominator by 3}\mright\rbrace \\ =\frac{5}{7} \\ \therefore5\colon7 \end{gathered}[/tex]Therefore, the answer is
Answer: 5:7
