john has decided to purchase a cellular phone, but he is confused about which rate plan to choose. The "occasional user" plan is $.20 per minute, regardless of how many minutes of airtime are used. The "frequent user" plan charges a flat rate of $40.00 per month for 180 minutes of airtime plus $.15 per minute for any time over 180 minutes.
Question: How much air time per month would john need to use before he upgrades from the occasional user plan to the frequent user plan?