the angle of triangle are x-40,x-20 and 1/2x+15

x-40+x-20+1/2x+15=180
2x+1/2x-60+15=180
4x+x/2=180+60-15
5x/2=225
x=225*2/5
x=450/3
x=90
so first angle = x-40=90-40=50
second angle=x-20=90-20=70
third angle=1/2*90+15=45+15=60
Now,50+70+60=180

hope it help