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Start your free trial###### Ivana Lescesen

19,442 Points# I do not understand the math? Thanks for your help

Can someone explain this math:

var randomNumber = Math.floor(Math.random() * (topNumber - bottomNumber + 1)) + bottomNumber;

## 3 Answers

###### codecorrupter

8,863 PointsFirst, you find the difference of the range of the numbers, and then +1. So, for the 10-25 example, 25 - 10 = 15. 15 + 1 = 16. Right now, we have Math.floor(Math.random() * 16). For that expression only, you'll get a random number from 1-15. 16 is exclusive. Then, you add the bottom number, and in this case, it would be 10. The math.floor runs first, and as you know, you can get 15 from just the math.floor expression, so if you add 10 (bottom number), there's the possibility to get the max number 25. If you get 1 for the math.floor expression and add 10, you can get the minimum of 10. I hope this makes sense.

###### Ivana Lescesen

19,442 PointsWhy do we add +1 ? If we get 1 + 10 would not that be eleven?

is this as same var dieRoll = Math.floor( Math.random() * (6 -1 +1) ) +1;

as this var dieRoll = Math.floor( Math.random() * 6) +1;

###### codecorrupter

8,863 Points+1 because of Math.floor. It rounds down. So if you want a number between 1-10, you need to +1 after the Math.floor, otherwise it would be 1-9. Consider this: Math.floor(Math.random() * 10); That expression is for 1-9. You need to +1 to make it, so you can get 10. Those examples you presented are indeed the same.

###### Ivana Lescesen

19,442 Pointsthank you :)