Jessica has the option to switch to a plan that charges $65.00 per month with unlimited text messages.
Jessica typically sends about 900 text messages per month.
Cost per message under the new plan = $65 / 900 = $ 0.072 per message
It make sense for her to switch to the new plan only if the cost per message in her existing plan is more than $ 0.072
Hope this helps..!!
Thank you :)