Answer :
[tex]\begin{gathered} \frac{robots}{\text{chess sets}}\colon\frac{3}{5}=\frac{21}{x} \\ \end{gathered}[/tex]
solve for x
[tex]\begin{gathered} 3x=21\times5 \\ 3x=105 \\ \frac{3x}{3}=\frac{105}{3} \\ x=35 \end{gathered}[/tex]the store sold 35 chess sets
and answer is: 21 robots + 35 chess sets = 56 toys sold by the store