Answer:
900 minutes
Step-by-step explanation:
Plan A: 700 free minutes + 15 cents per minute over 700 ($40)
Plan B: 500 free minutes + 5 cents per minute over 500 ($50)
The charges for x minutes (assumed x>700) for plan A will be
40+0.15(x-700)
The charges for x minutes (assumed x>500) for plan B will be
50+0.05(x-500)
To find the value of x where both charges are the same, we equate
40+0.15(x-700)=50+0.05(x-500)
Operating
40+0.15x-105=50+0.05x-25
Simplifying
0.10x=90
x=900 minutes
Both plans would charge the same amount when 900 minutes per month are used