Consider the enlargement of the rectangle. Use the proportion to find the missing dimension bod the original rectangle. (Sorry the pic is horrible)

Here's the right way:
[tex]\dfrac{x}{\frac 3 5} = \dfrac{20}{12}[/tex]
[tex]x = \dfrac{20}{12} \cdot \dfrac{3}{5} = 1[/tex]
Here's the way they want you to do it:
[tex]\dfrac{x}{\frac 3 5} = \dfrac{20}{12}[/tex]
[tex]12 x = 20 \times \dfrac{3}{5} = 12 \textrm{ FIRST BOX}[/tex]
[tex]x = 12/12 = 1 \textrm{ SECOND BOX}[/tex]