Considering Changing Her Cell Phone Plan

Please give me the step by step solution for this.

A real estate agent is considering changing her cell phone plan. There are three plans to choose from, all of which involve a monthly service charge of $20. Plan A has a cost of $.42 a minute for daytime calls and $.17 a minute for evening calls. Plan B has a charge of $.52 a minute for daytime calls and $.15 a minute for evening calls. Plan C has a flat rate of $80 with 275 minutes of calls allowed per month and a charge of $.38 per minute beyond that, day or evening.

Suppose that the agent expects both daytime and evening calls. At what point (i.e., percentage of total call minutes used for daytime calls) would she be indifferent between plans A and B? (Do not round intermediate calculations. Enter your answer as a percentage rounded to 2 decimal places. Omit the "%" sign in your response.)