Explanation:
Let x represent the number of months for the cell phone plan.
Since Lucy is contributing, Linus does not have to pay the full cost.
Instead Linus will have to pay:
($45 + $65x) - ($15 + $20x)
= $30 + $45x.
If x = 6,
Linus will pay $30 + $45(6) = $300.