Should You Accept a Discounted Annual Plan Instead of a Monthly One?

Suppose your TV or Internet provider offers you a discount if you prepay for a year, or for six months, instead of paying every month. A longer plan locks you into the service, so I wouldn't generally take it, but suppose that weren't a consideration, would you save money by going for the annual plan? How much discount must you get to save money with the annual plan?

This spreadsheet tells you:

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If the embed doesn't work, click here. This will also let you make a copy of the spreadsheet to change the parameters and try different scenarios.