let the last vertex of the shaded triangle be y

Triangle BYC area= 1/2bh (General formula)

1/2(2n)*h=80

2n*h=160

2nh=160

nh=80

Consider triangle ABC

area=1/2(3n)*h

1/2*3nh

we know nh=80

1/2*3(80)=120

so triangle ABC area=120

Also triangle ABY=120-80=40