C1: [TEX]\frac{x}{3}= \frac{y}{2} = \frac{x-y}{3-2}=20[/TEX]
[TEX]\Rightarrow x=20.3= \fbox{60}, \ \ y=20.2= \fbox{40}[/TEX].
C2: [TEX]\frac{x}{3}= \frac{y}{2}=k \Rightarrow x=3k, \ y=2k[/TEX].
[TEX]\Rightarrow x-y=3k-2k = k = 20 \Rightarrow x=60, \ y=40[/TEX].
C3: [TEX]\frac{x}{3}= \frac{y}{2} \Rightarrow 2x=3y \Rightarrow 2x-2y=3y-2y[/TEX]
[TEX]\Rightarrow 2(x-y)=y \Rightarrow y=40 \Rightarrow x=60[/TEX].