For the family plan:
Let:
y1 = Total cost
a1 = Charge for each device = 40
b1 = Monthly fee = 90
P = Number of devices
So:
The model for the family plan will be given by:

For the Mobile share plan:
Let:
y2 = Total cost
a2 = Charge for each device = 35
b2 = Monthly fee = 120
P = Number of devices
The model for the mobile share plan will be given by:

We can find the number of devices that would make the two plans equal in cost. Solving for:

The number of devices that would make the two plans equal in cost is 6.
If P = 3

For 3 devices The family plan cost $210 and for 3 devices the mobile share plan cost $225, Therefore we can conclude that for 3 devices The family plan is more affordable.