# The cost of printing the school newspaper is $500 for 800 copies and$620 for 1200 copies. If the printing cost is a linear function of the copies printed, how do you find the cost of printing 1500 copies?

Apr 27, 2017

Cost of printing $1500$ copies is  $710  #### Explanation: This is a linear function, i.e obeys straight line equation: Let y denotes cost in $ and x denotes number of copies.
Slope $m = \frac{620 - 500}{1200 - 800} = \frac{120}{400} = \frac{3}{10}$
Equation of straight line is $y = m x + c \mathmr{and} 500 = \frac{3}{10} \cdot 800 + c \mathmr{and} 500 = 240 + c \mathmr{and} c = 500 - 240 = 260$
Equation of straight line is $y = \frac{3}{10} x + 260$
Check: $620 = \frac{3}{10} \cdot 1200 + 260 \mathmr{and} 620 = 360 + 260 \mathmr{and} 620 = 620$
Cost of printing $1500$ copies is  y =3/10*1500+260 = 450+260 = \$710  graph{3/10 x+260 [-1280, 1280, -640, 640]} [Ans]