What we know is that a 6 foot tall tent casts a 15 foot shadow. Lets represent that given this way: 6 = 15
So, the unknown (y), or what we want to know, is how tall the cardboard box if it has a 6 foot long shadow (noticeably the same height as the tent):
y = 6
6 = 15
y = 6
(6 x 6) / 15 = y
36 / 15 = y
2.4 = y
The height of the cardboard box is 2.4 feet.
