Question 174355
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We'll interpolate the given data to get monthly cost for 54 minutes, which represented by  {{{x}}}:
{{{system(30minutes=15.46,54minutes=x,84minutes=20.32)}}}
{{{(30-54)/(30-84)=(15.46-x)/(15.46-20.32)}}}
{{{(-24)/(-54)=(15.46-x)/-4.86}}}, cross multiply
{{{-24(-4.86)=-54(15.46-x)}}}
{{{116.64=-834.84+54x}}}}
{{{116.64+834.34=54x}}}
{{{54x=951.48}}} ------->{{{cross(54)x/cross(54)=cross(951.48)17.62/cross(54)}}}
{{{highlight(x=17.62)}}}, Monthly cost for 54 minutes.
Thank you,
Jojo</pre>