You and three friends are preparing a mailing for your organization. There are four steps in the process and each one of you will perform a step and each step will be performed by only one of your group. After doing this for twenty minutes, you want to determine how many envelopes can be completed in the next hour. The steps are as follows: fold the insert which takes 15 seconds, place the insert into the envelope and seal the envelope which takes 20 seconds, place the mailing labels on the envelope which takes 30 seconds, stamp the envelope and ensure everything is correct which takes 28 seconds.

How many envelopes can your group complete in one hour?