Since the triangles are similar, the side lengths are proportional.
In other words, if a 50 foot building makes a 75 foot shadow, that is a ratio of 50/75, or 2/3 if you reduce it.
This is the same ratio that describes Sarah's height, S, compared to her shadow, which the problem says is 6 feet long.
So 2/3 = S / 6
You can cross multiply to find S... 12 = 3S..... so S = 4 feet tall... Sarah is 4 feet tall.
