Remember that if there is a proportional relationship between two quantities, their relationship can be represented by an equation of the form $y = k x$. Sometimes writing an equation is the easiest way to solve a problem.

For example, we know that Denali, the highest mountain peak in North America, is 20,310 feet above sea level. How many miles is that? There are 5,280 feet in 1 mile. This relationship can be represented by the equation

$\displaystyle f=5,280 m$

where $f$ represents a distance measured in feet and $m$ represents the same distance measured in miles. Since we know Denali is 20,310 feet above sea level, we can write

$\displaystyle 20,310=5,280 m$

Solving this equation for $m$ gives $m = \frac{20,310}{5,280}≈ 3.85$ , so we can say that Denali is approximately 3.85 miles above sea level.