Answer :
A 1min = $0.10
A 100min = $0.10 x 100 = $10
B 100min = $12
B-A = $2
101th min onwards:
A 1 min = $0.10
B 1 min = $0.08
A-B = $0.02
$2 / $0.02 = 100 min
A 200 min = $0.10 x 200 = $20
B 200 min = $12 + ($0.08 x 100) = $20
A 201 min = $20.10
B 201 min = $20.08
Monthly talking time should be 201 minutes and above for B to be the better choice
A 100min = $0.10 x 100 = $10
B 100min = $12
B-A = $2
101th min onwards:
A 1 min = $0.10
B 1 min = $0.08
A-B = $0.02
$2 / $0.02 = 100 min
A 200 min = $0.10 x 200 = $20
B 200 min = $12 + ($0.08 x 100) = $20
A 201 min = $20.10
B 201 min = $20.08
Monthly talking time should be 201 minutes and above for B to be the better choice
x - talking time in minutes
y - price
A:
[tex]y=0.1x[/tex]
B:
[tex]y=12+(x-100)\cdot0.08 \\ y=12+0.08x-8 \\ y=0.08x+4[/tex]
B price must be less than A so:
[tex]0.08x+4 < 0.1x \\ 4 < 0.1x-0.08x \\ 4 < 0.02x \\ 200 < x \\ x>200[/tex]
More than 200 minutes of talking would make B the better choice.
y - price
A:
[tex]y=0.1x[/tex]
B:
[tex]y=12+(x-100)\cdot0.08 \\ y=12+0.08x-8 \\ y=0.08x+4[/tex]
B price must be less than A so:
[tex]0.08x+4 < 0.1x \\ 4 < 0.1x-0.08x \\ 4 < 0.02x \\ 200 < x \\ x>200[/tex]
More than 200 minutes of talking would make B the better choice.