well, if we divide 21 by 6, we get a quotient of 3 and a remainder of 3, let's use those in the mixed fraction whilst retaining the denominator of 6.
[tex]\cfrac{21}{6}\qquad \qquad 21\div 6\qquad \stackrel{quotient}{3}\qquad \stackrel{remainder}{\underline{3}}\qquad \implies\qquad 3\frac{\underline{3}}{6}\implies \stackrel{simplified}{3\frac{1}{2}}[/tex]
