Question

The cost of two different mobile phone contracts:
Company A: $30 and $0.15 per minute for phone calls
Company B: $10 and $0.55 per minute for phone calls
When is company A cheaper than company B?

Answer 1

holy crap I had this same problem in MTH 163.
Comp. A 0.15x+30
Comp. B 0.55x + 10
0.15x+30=0.55x+10
30=0.40x+10
20=0.40x
x=50
so at 50 minutes they are equal.
lets test 45 minutes..
0.15(45)+30=36.75
0.55(45)+10=34.75
at less than 50 minute company B is cheaper.
so it has to be x>50 for company A to be cheaper.

Answer 2

the function is
A(t) = 30 + 0.15t
B(t) = 10 + 0.55t

t = minutes

when A costs as much as B :
A = B
30 + 0.15t = 10 + 0.55t
20 = 0.4t
t = 50

then plug any number that's less than 50, let's plug 1
A(1) = 30 + 0.15 = 30.15
B(1) = 10 + 0.55 = 10.55

That means, when the t<50 , A costs more than B.
Therefore A is cheaper then B when t>50

Answer 3

prettier way of explaining it :P