For the statement "Math.floor(Math.random() * (6 - 1 + 1)) + 1;" why am I subtracting 1 from 6 and then adding 1?

Not sure I understand your question. To get a random number between 1 and 6 you can do this:

Math.floor((Math.random() * 6) + 1);

You need to add the 1 because the random() function returns a number between 0 (inclusive) and 1 (exclusive).

The formula you are questioning is for cases where the lower bound is not 1.

So if you want a random number between 50 and 75 you need

Math.floor((Math.random() * (75 - 50 + 1) + 50);

Here you are multiplying the random number by 26 (which gives a random number between 0 and 25, and then you are adding 50 to that number, giving a random number between 50 and 75.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random

According to MDN, it's a case of whether you want to make the max number INclusive or EXclusive: 1.) Returns a random integer between min (included) and max (excluded)

function getRandomInt(min, max) {
  return Math.floor(Math.random() * (max - min)) + min;
}

2.) Returns a random integer between min (included) and max (included)

function getRandomIntInclusive(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

So the formula that they provide for this challenge already sets you up to have the maximum number included in the possible outcomes of the random number generated, as the +1 indicates. - I was thrown at first too.. better formula to have been given first off would have been the one stated on MDN, then the min and max and +1 are clear in why each are being used.

The formula is built to work for any upper and lower values, not just 1 for the lower value. If you wanted a random number between 50 and 75 you wouldn't be able to just multiply by 75.

Your questions and Answers were useful. Thanks all

The code need to be something like this:

function getRandom (lower, upper) {
    return Math.floor((Math.random() * (upper - lower + 1) + lower);
}

console.log( getRandom(10, 20) );

I guess I understood. Thanks to jcorum. Forget about the (6 -1 +1 ). Let's change the 6 to up and -1 to down, then we have (up - down +1). The 6 and 1 could be any other number, but the +1 need to always be there, cause it is the formula. Is it right?