in short, we divide 40 by (1+3) and distribute accordingly.
[tex]\bf 40\qquad 1:3\qquad \cfrac{1}{3}~\hspace{5em}\cfrac{1\cdot \frac{40}{1+3}}{3\cdot \frac{40}{1+3}}\implies \cfrac{1\cdot 10}{3\cdot 10}\implies \cfrac{10}{30}\implies 10:30[/tex]
