Answer :
First, let's convert the mixed number into a fraction:
[tex]1\frac{2}{3}=1+\frac{2}{3}=\frac{3}{3}+\frac{2}{3}=\frac{5}{3}[/tex]Then, we can use the following rule of three:
[tex]\begin{gathered} 6\text{ toy cars}\to\frac{2}{5}\text{ of collection} \\ x\text{ toy cars}\to\frac{5}{3}\text{ of collection} \\ \\ \frac{6}{x}=\frac{\frac{2}{5}}{\frac{5}{3}} \\ \frac{6}{x}=\frac{2}{5}\cdot\frac{3}{5} \\ \frac{6}{x}=\frac{6}{25} \\ x=25 \end{gathered}[/tex]Therefore the answer is 25 toy cars.