Question

Joylin is writing an equation to model the proportional relationship between y, the total cost in dollars of downloading videos from a website, and x, the number of videos downloaded. She knows that the total cost to download 3 videos was $12. Her work to find the equation is shown below.

Joylin’s Work

Step 1
k = StartFraction 3 over 12 EndFraction = 0.25
Step 2
y = 0.25 x

Where did Joylin make her first error?
Joylin divided 3 and 12 instead of multiplying the two numbers.
Joylin computed the decimal equivalent of StartFraction 3 over 12 EndFraction incorrectly.
Joylin used 0.25 as the coefficient of x instead of a product of x and y.
Joylin switched the numerator and the denominator when she found k.

Answer 1

Joylin switched the numerator and the denominator when she found k.

Explanation: For y = kx, k = y/x = 12/3 = 4, so the correct equation is y = 4x. Joylin used 3/12 (0.25) instead of 12/3.