ANSWER
a) C = 24 + 0.10x
b) $76.60
Step-by-step explanation
Her plan has a monthly fee and charge.
Let the monthly fee be M
Let the cost per minute be n.
The cost of each month will be
Cost = Monthly fee + (number of minutes * cost per minute)
C = M + (n * x)
C = M + nx
We have that in June, she used 260 minutes and the cost was $50. This means that:
50 = M + 260 * n
50 = M + 260n ____(1)
In July, she used 1000 minutes and the cost was $124. This means that:
124 = M + 1000 * n
124 = M + 1000n ____(2)
We have to solve these simultaneously for M and x:
M + 260n = 50 ____(1)
M + 1000n = 124 ___(2)
From (1):
M = 50 - 260n
Put that in (2):
50 - 260n + 1000n = 124
50 + 740n = 124
740n = 124 - 50 = 74
n = 74 / 740
n = $0.10
Recall that:
M = 50 - 260n
M = 50 - 260 * 0.10 = 50 - 26
M = $24
This means that the monthly fee is $24 and the cost per minute is $0.10.
Put that in the initial equation, this means that the Monthly Cost is:
C = 24 + 0.10x
b) In August, she used 526 minutes of calling time. This means that:
x = 526
Her bill that month was therefore:
C = 24 + 0.10 * 526
C = 24 + 52.6
C = $76.60
Her bill that month was $76.60