Answer:
Company one charges $11 + $0.16 per min.
Then if you talk for x minutes, the cost will be:
C₁(x) = $11 + ($0.16 per min)*x
For company two, the prize is $20 + $0.11 per min, and if yo talk for x minutes, the cost will be:
C₂(x) = $20 + ($0.11 per min)*x
Now we want to find the value of x, the number of minutes, such that the cost is the same with both companies.
C₁(x) = C₂(x)
$11 + ($0.16 per min)*x = $20 + ($0.11 per min)*x
($0.16 per min)*x - ($0.11 per min)*x = $20 - $11
($0.05 per min)*x = $9
x = $9/($0.05 per min) = 180 mins
If you speak for 180 minutes, the cost is the same in both companies.
