I did consider this, and it seemed to make it feel a little clearer—but when I first posted, I didn't feel it was obvious that the fraction of the rectangle occ

I did consider this, and it seemed to make it feel a little clearer—but when I first posted, I didn't feel it was obvious that the fraction of the rectangle occupied by the triangle would remain constant if you stretched it. But I think the other replies prove that that is true. Thank you.