The required cost is $ 101.22.
This problem can be solved by unitary method.
Given that monthly cost of customer if he uses 410 minutes is $71.
So, cost per minute = [tex]\frac{71}{410}[/tex]
=$ 0.17
And if the customer uses 720 minutes then the cost is $118.
So, cost per minute is = [tex]\frac{118}{720}[/tex]
=0.16
Now if he uses 687 minutes then for 410 minutes cost incurred will be
= 410 × 0.17
= $ 69.7
And for next 197 minutes cost incurred will be = 197 × 0.16
= $ 31.52
Total cost for 687 minutes = $(69.7 + 31.52)
= $ 101.22
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