Question

A wall's height is two-thirds that of its length. The entire wall will be painted. The paint costs \(\$ 12\) per gallon, and 1 gallon of paint covers 60 square feet. What expression gives the cost of the paint (assuming one can purchase partial and full gallons) to cover a wall that is \(\mathrm{L}\) feet long? (A) \(\quad\) Cost \(=L^{2}\) (B) \(\quad\) Cost \(=\frac{1}{5} L^{2}\) (C) \(\quad\) Cost \(=\frac{2}{15} L^{2}\) (D) \(\quad\) Cost \(=\frac{3}{64} L^{2}\)

Short answer

Option (C): \( \frac{2}{15}L^2 \).

Step-by-step solution

Determine the height of the wall

The height of the wall is described as two-thirds of its length. If the wall's length is \( L \), the height is \( \frac{2}{3}L \).

Calculate the area of the wall

The area of a rectangular wall is given by the formula \( \, \text{Area} = \text{length} \times \text{height} \, \). Therefore, the area of the wall is \( L \times \frac{2}{3}L = \frac{2}{3}L^2 \).

Determine gallons of paint needed

Since 1 gallon of paint covers 60 square feet, the number of gallons needed is the total area divided by coverage per gallon, i.e., \( \frac{\frac{2}{3}L^2}{60} \). Simplifying this gives \( \frac{L^2}{90} \).

Calculate the cost of the paint

The cost of the paint is the number of gallons needed multiplied by the cost per gallon. The expression for the cost is \( \frac{L^2}{90} \times 12 = \frac{L^2}{7.5} \), which simplifies to \( \frac{2}{15}L^2 \).

Identify the correct answer choice

Option (C) \( \frac{2}{15}L^2 \) matches our derived expression for the cost of the paint.

Key concepts

Geometry

Geometry plays a crucial role in understanding the spatial aspects of different objects, such as a wall in this exercise. Geometry helps us comprehend dimensions, shapes, and the area of constructs we encounter in mathematical problems.
When determining the dimensions of the wall, we start with its length, denoted as \( L \). The height of the wall is then described as \( \frac{2}{3}L \), showing a clear relationship between length and height. This proportionality is a fundamental concept in geometry. It illustrates how one dimension can influence another, which is a common occurrence in real-life scenarios as well.
In this exercise, understanding this concept allows us to calculate the wall's area, a key step in determining the cost of materials needed. To find the area, we use the formula for the area of a rectangle: length times height. Therefore, the area of the wall is given by \( L \times \frac{2}{3}L \), which computes to \( \frac{2}{3}L^2 \).
Recognizing this geometric relationship enables us to solve problems related to coverage and cost, forming a basis for further algebraic calculations.

Mathematical Expressions

Mathematical expressions are equations or formulas that represent relationships or operations on quantities. They allow us to model and solve real-world problems efficiently.
In this problem, the mathematical expression for the area \( \frac{2}{3}L^2 \) is derived from geometric measurements. This expression helps in the subsequent calculations, such as determining the amount of paint needed. Each part of the expression holds a meaning—a reflection of physical dimensions transcribed into mathematical language.
Using expressions simplifies complex scenarios, translating them into manageable parts. When considering the paint coverage, we formulated \( \frac{\frac{2}{3}L^2}{60} \) as an expression to calculate the gallons needed. This expression represents the relationship between the area to be painted and the coverage per gallon. Simplifying this expression leads us to \( \frac{L^2}{90} \), making it easier to handle computationally.
Mathematical expressions provide clarity and precision, essential for understanding and solving problems systematically.

Algebraic Manipulation

Algebraic manipulation involves rearranging and simplifying expressions to solve equations or make calculations simpler and clearer. This process is a vital skill in handling mathematical problems efficiently.
In this exercise, after determining the expression for the number of gallons of paint, \( \frac{L^2}{90} \), we move on to calculating the cost. The manipulation involves multiplying this expression by the cost per gallon, \( 12 \), yielding \( \frac{L^2}{90} \times 12 \).
Careful simplification is crucial as it reduces the expression to \( \frac{L^2}{7.5} \), which can be further simplified to \( \frac{2}{15}L^2 \). This final expression represents the total cost of the paint in a concise and clear form.
By algebraically manipulating expressions, we attain results that are not only precise but also optimized for problem-solving. These skills are indispensable for efficiently progressing through mathematical exercises and real-world applications.