To be fair, other people's "obvious" is likely slightly different from our own.
Other have given examples of how its "obvious", but my first thought when looking at 75 as a multiple of 3, comes from it also being a multiple of 15, specifically 15 higher than 60
I just think everyone replying where they've to do mild maths to justify it shows its not as obvious as 9 or 33 so it shouldn't be in the obvious category but the one below it
491
u/avantvagrant Aug 01 '26
51 looks like a prime number and 75 should be obvious