You can write the proportion
[tex]20\div \dfrac{3}{5}=x\div 1[/tex]
In fact, if she waxes the floor at a constant rate, the amount of floor waxed and the time spent will be proportional.
Solving for x, we have
[tex]\dfrac{3}{5}x=20 \iff x=20\cdot\dfrac{5}{3}=\dfrac{100}{3}[/tex]