Question

Nicole's cleaning company uses a linear model to determine the amount they charge their clients. They charged $144 for a cleaning job that took 4 hours. They charged $188 for a cleaning job that took 6 hours. Which equation could be used to determine the amount charged, C, for a cleaning job that takes t hours?A.C = 10t + 104B.C = 36t C.C = 44t + 56D.C = 22t + 56

Answer 1

D

Explanation:

Let the following point represent the first job: (4, $144). Let (6, $188) represent the second. Then we find the equation of the line through these two points. As we go from (4, $144) to (6, $188), x increases by 2 and y increases by $44. Thus, the slope of this line is m = rise/run = $44/(2 hrs), or $22/hr.

Using the slope-intercept formula for a straight line, y = mx + b, we find the equation of this line by finding the y-intercept, b:

Subbing the given info ($22/hr for m, 4 hr for x and $144 for y), we get:

$144 = ($22/hr)(4 hr) + b, or

$144 = $88 + b, which yields b = $56.

Thus, the desired equation is y = mx + b, or y = ($22/hr)x + $56. This matches the last answer (D)