Final answer:
To buy the headphones, Lea needs to wash a minimum of 15 cars.
Step-by-step explanation:
To determine the least number of cars Lea needs to wash in order to buy the headphones, we need to find the difference between the cost of the headphones and the amount of money she has already saved. Lea needs at least $240 and has saved $30, so she still needs to save $240 - $30 = $210.
Since Lea earns $14 per car she washes, we can set up an equation to represent the remaining amount of money she needs to save: $14x = $210, where x is the number of cars Lea needs to wash.
To solve for x, we divide both sides of the equation by $14: x = $210/$14 = 15. Therefore, the least number of cars Lea needs to wash to buy the headphones is 15.