The difference in cost between the two plans after 18 months is $240.
How to calculate the difference in cost between the two plans
To calculate the difference in cost between the two plans after 18 months, find the total cost for each company at the 18th month.
For Company A, the cost increases by $35 per month. So, at the 18th month:
Total cost for Company A = 180 + (18 - 4) * 35 = 180 + 14 * 35 = 180 + 490 = $670
For Company B, the cost increases by $50 per month. So, at the 18th month:
Total cost for Company B = 210 + (18 - 4) * 50 = 210 + 14 * 50 = 210 + 700 = $910
Now, find the difference in cost between the two plans after 18 months:
Difference in cost = Total cost for Company B - Total cost for Company A
Difference in cost = $910 - $670 = $240
Therefore, the difference in cost between the two plans after 18 months is $240.