How long is the shadow cast by a tree 21 m high, if a person 1.75 m tall gives a shadow 3 m long?

To solve this problem, you need to make a proportion in which the height of a tree (21 meters) relates to the length of its shadow (x) in the same way as a person’s height (1.75 m) relates to the length of his shadow (3 meters).

We get:

21 / x = 1.75 / 3.

We multiply the extreme terms of the proportion among ourselves.

Will be:

1.75 * x = 21 * 3.

1.75 * x = 63.

x = 63 / 1.75 = 36 meters.

Answer: A tree with a height of 21 meters casts a shadow 36 meters long.