Hi Lily,

I think you are looking for an equivalent fraction, that is you want to know the denominator so that $\large \frac{20}{???}$ is equivalent to $\large \frac13.$ One way to find equivalent fractions is to take a fraction and multiply the numerator and denominator by the same amount. Thus $\large \frac13$ is equivalent to $\large \frac{1 \times 2}{3 \times 2}$ which is $\large \frac26.$ For another example the fraction $\large \frac35$ is equivalent to $\large \frac{3 \times 4}{5 \times 4}$ which is $\large \frac{12}{20}.$

Can you now see how to find a fraction with numerator 20 and equivalent to $\large \13?$

Write back if you need more help,
Penny