So for the following question, I got 6 switches (well, 5.025883189, but rounded up to the next whole number/whole switch), but my professor got 5 switches. I think it's because he changed the numbers for the question he assigned to us, but the answer he is using for his key is from an identical problem in our book with slightly different numbers.

Assume that a electronic switch operates properly 60% of the time. What is the smallest number of switches that can be wired in parallel to ensure the system functions (at least one switch works) 99% of the time