The function C(m) = 49.99m represents the total cost of Josie's cell phone bill for months. She decides to switch to a new company that offers a better data plan, but changes a higher monthly rate. Her new phone plan is modeled by the function C(m) = 64.99m.

Write a function D(m) that models the difference in the cost between the two cell phone plans after m months.
D(m) =

After 3 months, how much more has Josie paid for her new plan?