Benton has an extension ladder than can only be used at a length of 10 feet, 15 feet, or 20 feet. he places the base of the ladder 6 feet from the wall and needs the top of the ladder to reach 8 feet. which ladder length would benton need to use to reach this height on the wall?

10 feet.

Think of it like this:

The wall is upright, it's 90° and it's 8 feet high. The distance from the bottom of the ladder to the bottom of the wall (along the ground, which is perpendicular to the wall) is 6 feet. When Benton places the base of the ladder 6 feet away from the wall and the top is resting at the top of the wall, it looks like a triangle, right?

Using pythagoras (this rule about right-angled triangles and stuff), we can already see that the two sides when simplified are 3:4. Because the 'triangle' is a right-angled triangle, the other side HAS to be 5 to complete the ratio. We just multiply it by 2 to get the correct ratio and that's your answer!