model the following problem please

a You have been approached by three telephone companies to subscribe to their long-distance set-vice in the United States. MaBell will charge a flat $16 per month plus $.25 a minute. PaBell will charge $25 a month but will reduce the per-minute cost to $.21. As for BabyBell, the flat monthly charge is $18, and the cost per minute is $.22. You usually use an average of 200 minutes of long-distance calls a month. Assuming that you do not pay the flat monthly fee unless you make calls and that you can apportion your calls among all three companies as you please, how should you use the three companies to minimize your monthly telephone bill