Petra has the choice of two mobile phone plans. Unlonely Planet costs $50 per month and charges 45c per call and Optel costs $70 per month and charges 25c per call. Graph the charges up to $150 for both plans on the same axes and use your graph to determine how many calls she would have to make to be better off under Unlonely planet.

Solution:

Number of call: x

Unlonely Planet: y =50+0.45x

Optel: y=70+0.25x

50+0.45x>70+0.25x

x>100

So when there are more than 100 calls, we use Optel

When there are less than 100 calls, we use Unlonely Planet