If r varies jointly as s and t and inversely as u, and r = 18 when s = 2, t = 3, and u = 4, find s when r = 6, t= 2 and u = 4.

Lin 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= 3/12= 0.25 step 2 y= 0.25x where did joylin make her first error?