A flagpole casts a shadow that is 18 feet long.

It is 30 feet from the top of the flagpole to the top of the shadow.
How tall is the flagpole?

Apply Pythagoras's theorem:

h = √ ( 30² - 18² ) = √ ( 900 - 324 ) = √ 576 = 24 ft

To find the height of the flagpole, let's use similar triangles.

We have two right triangles here - one formed by the flagpole, its shadow, and a vertical line connecting the top of the flagpole to the top of the shadow, and the other formed by the vertical line, the shadow, and the ground.

Since the two triangles are similar, the ratios of their corresponding sides will be equal. Let's consider the ratio of the height of the flagpole to the length of its shadow.

Let's call the height of the flagpole "x" feet.

So, we have the following proportion:

x / 18 = (x + 30) / 18

To solve for x, we can cross-multiply:

18(x + 30) = 18x

Distribute 18 on the left side:

18x + 540 = 18x

Subtract 18x from both sides to get rid of the x term:

540 = 0

At this point, we realize that the equation does not have a solution. This means that there is no possible value for the height of the flagpole that satisfies the given conditions. It could be an error in the measurements or a problem with the question itself. In any case, without additional information or clarification, we cannot determine the height of the flagpole.