I don't think the 10 year part is right. After one year 96% of people haven't had a failure. By year two 96% of those remaining so it goes 0.96^10 = 0.66 or 34% failure rate in 10 years in your worst case and 0.98^10 = 0.82 or 18% for your best case.

Also note: A 2% or 4% chance of pregnancy is pretty good odds using only condoms for 1 year. If using only condoms for 10 years that number is creeping up on 20% or 40%.

This is a weird one as well because you aren’t taking into account fertility. Say someone gets into a serious relationship at 25, if your’re a women your fertility starts to decrease slightly in your late 20s and even more so after you hit 35.

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