Fruit Smoothie Calculation

A recipe for a fruit smoothie uses a total of 4 cups of fruit, which consists of apples and bananas. If the number of apples used is 3 times the number of bananas, how many cups of bananas are used in the smoothie?

Let $x$ represent the number of cups of bananas used. Then the number of cups of apples can be represented as $3x$. The equation that represents the total amount of fruit used can be expressed as:

$$x + 3x = 4$$