The subtraction in the code

It would be easier if you understand this with the help of an example first..then visualize what is going on...if upperVal = 20 and lowerVal = 12, then we need to find a random number in the range [12,20]....when we subtract 12 from 20 we get 8, then we add 1 which gives us 9... So now

Math.floor(Math.random() * (upperVal - loweVal +1 ))

If this code runs we get a random number from 0 to 8 both included, now what was our initial problem? We wanted to get a random number between 12 and 20, but we are getting value between 0 and 8, we consider the extreme cases first, Suppose the above code gives us 0, so if we add 12 (lowerVal) to 0 we get our result...now if the above code gives 8 and we add 12 to it, we get 20 which is what we wanted... all the other case will also be in the range [12,20]. So add lowerVal to the above code and u will get the random number in the range.