I only suggested something you had already tried, but thanks, here's something else you might have already tried...  20C = 68F, exactly.  75F is not going to be an exact match, and I'm not going to do the math right now, but if you don't know how to do the math, you can type "convert 75F to C" in Google and it will tell you that it is 23.8888889C (which may or may not be 23.8 repeating rounded up).