<PRE><B>Question 1134131</B>
let x be the total number of minutes of calls in a month 
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Let C(A) be the cost of plan A and C(B) be the cost of plan B
:
C(A) = 5 +0.10x
:
C(B) = 8 +0.07x
:
For this problem, we want C(A) = C(B)
:
5 +0.10x = 8 +0.07x
:
0.03x = 3
:
x = 3/0.03 = 100 
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100 minutes of calls in a month results in C(A) = C(B)
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check the answer
:
C(A) = 5 +0.10 * 100 = $15
:
C(B) = 8 +0.07 * 100 = $15
:
C(A) = C(B) for 100 minutes in a month
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