The interior angles of a triangle give 180° when summed. So, we have
[tex](x-40)+(x-20)+(\frac{x}{2}-10)=180\iff \frac{5}{2}x-70=180 \iff \frac{5}{2}x=250 \iff 5x=500 \iff x=100[/tex]
So, if x is 100, the three angles are
[tex]100-40=60[/tex]
[tex]100-20=80[/tex]
[tex]\frac{100}{2}-10=40[/tex]
