The spring and fall changes between standard and daylight savings time create an interesting problem for telephone bills. 

In the spring, this switch occurs at 2:00 a.m. on a Sunday morning (late March, early April) when clocks are reset to 3:00 a.m. The symmetric change takes place usually on the last Sunday in October, when the clock changes from 2:59:59 back to 2:00:00.

Develop equivalence classes for a long-distance telephone service function that bills calls using the following rate structure: 

Call duration ≤20 minutes charged at $0.05 per minute or fraction of a minute

Call duration >20 minutes charged at $1.00 plus $0.10 per minute or fraction of a minute in excess of 20 minutes. 

Make these assumptions:

 •Chargeable time of a call begins when the called party answers, and ends when the 

•Calling party disconnects. 

•Call duration of seconds are rounded up to the next larger minute. 

•No call lasts more than 30 hours.