Question

The monthly cost of water use is a linear function of the amount of water used (HCF). The cost for using 16 HCF of water is $39.04, and the cost for using 44 HCF is $79.64. What is the cost for using 22 HCF?

Answer 1

• The cost for using 22 HCF is $47.74

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Equation of line:

• y = mx + b, where m- slope, b- y-intercept

Given values can be interpreted as pairs of coordinates of the two points:

• (16, 39.04) and (44, 79.64)

Find the slope of the line:

• m = (79.64 - 39.04)/(44 - 16) = 40.6/28 = 1.45

Find the y-intercept, by substituting coordinates of one point:

• 39.04 = 1.45*16 + b
• 39.04 = 23.2 + b
• b = 39.04 - 23.2
• b = 15.84

The equation becomes:

• y = 1.45x + 15.84

Find the value of y when x = 22:

• y = 1.45*22 + 15.84 = 31.9 + 15.84 = 47.74