At a certain time of day, a person 6 feet tall casts a 4-foot shadow. how long is the shadow cast by a 21-foot tree at the same time?

14ft

Explanation: here we may assume a linear relationship.

Where S, the shadow, is equivalent to H, the height.

and the relation is S = K*H

where K is a constant.

So if a person 6 feet tall casts a 4-foot shadow, we have:

4 = K*6

now we need to isolate K by dividing both sides by 6.

K = 4/6 = 2/3

Now that we know the value of K, if we want to find the shadow casted by a 21 ft tree, we need to solve:

S = (2/3)*21 = 2*(21/3) = 2*7 = 14

So the shadow casted is of 14ft