2022 AMC 10A Problems/Problem 4

Problem

In some countries, automobile fuel efficiency is measured in liters per $100$ kilometers while other countries use miles per gallon. Suppose that 1 kilometer equals $m$ miles, and $1$ gallon equals $l$ liters. Which of the following gives the fuel efficiency in liters per $100$ kilometers for a car that gets $x$ miles per gallon?

$\textbf{(A) } \frac{x}{100lm} \qquad \textbf{(B) } \frac{xlm}{100} \qquad \textbf{(C) } \frac{lm}{100x} \qquad \textbf{(D) } \frac{100}{xlm} \qquad \textbf{(E) } \frac{100lm}{x}$

Solution 1

The formula for fuel efficiency is $\frac{\text{Distance}}{\text{Gas Consumption}}.$ Note that $1$ mile equals $\frac 1m$ kilometers. We have $\frac{x\text{ miles}}{1\text{ gallon}} = \frac{\frac{x}{m}\text{ kilometers}}{l\text{ liters}} = \frac{1\text{ kilometer}}{\frac{lm}{x}\text{ liters}} = \frac{100\text{ kilometers}}{\frac{100lm}{x}\text{ liters}}.$ Therefore, the answer is $\boxed{\textbf{(E) } \frac{100lm}{x}}.$

~MRENTHUSIASM

Solution 2

Since it can be a bit odd to think of "liters per $100$ km", this statement's numerical value is equivalent to $100$ km per $1$ liter:

$1$ km requires $l$ liters, so the numerator is simply $l$ . Since $l$ liters is $1$ gallon, and $x$ miles is $1$ gallon, we have $1\text{ liter} = \frac{x}{l}$ .

Therefore, the requested expression is $100\cdot\frac{m}{(\frac{x}{l})} = \boxed{\textbf{(E) } \frac{100lm}{x}}.$ -Benedict T (countmath1)

Solution 3

A car that gets a gallon for $x$ miles has a fuel efficiency of $\frac{1 \text{ gallon}}{x\text{ miles}}.$ We want to convert these units to liters and kilometers. Since one gallon is $l$ liters, that can be rewritten as $\frac{l \text{ liters}}{x\text{ miles}}$ .

Now to convert the miles into kilometers. $1$ km is $m$ miles, so $1$ mile is $\frac{1}{m}$ km. Plug that in to get $\frac{l \text{ liter}}{x * \frac{1}{m} \text{ miles}}.$ Simplifying, we get $\frac{lm \text{ liters}}{x \text{ miles}}$ .

But the problem asked for the fuel efficiency of liters per $100$ km, not just $1$ km. So instead of $m$ , we put in $100 m$ , getting $\boxed{\textbf{(E) } \frac{100lm}{x}}$ as the answer.

~mihikamishra

Video Solution 1 (Quick and Easy)

https://youtu.be/JX4u3V2IqY0

~Education, the Study of Everything

Video Solution 2

https://youtu.be/qACAjp1HSxA

2022 AMC 10A (Problems • Answer Key • Resources)
Preceded by Problem 3	Followed by Problem 5
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 10 Problems and Solutions

Page

Toolbox

Search