Let's pretend the number is 15, so 1 less than "15", if you think about it, would be 14, and to get there, you do 15 - 1, now let us switch "15" back to a number "n", and now we have "n - 1"
It’s n-1 because 1 less than a number(n) would be n-1 and you may think what about 1-n, the words for that is one Minus a number or a number less than 1 but since it’s not that the answer is N-1
