QR Probability equations

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Can somebody explain when to use what equations with what scenario?

* n!/(n-r)!

* n!/r!(n-r)!

* n!

I was going through DAT Destroyer and I can't tell when to use what equation.

 

[Streetwolf]

Use the first when you need to choose r objects from a total of n objects and order DOES matter.

Example: You need to choose 3 winners out of 20 people who will receive prizes. The first place winner gets $1,000; the second place winner gets $500; the third place winner gets $250. Obviously the order in which you choose them is important since there are different prize levels.

Use the second when you need to choose r objects from a total of n objects and order DOES NOT matter.

Example: You need to choose 3 winners out of 20 people who will receive prizes. Each prize is worth $500. Since the prize is the same, it does not matter how the three winners are chosen. The question could also ask how many ways to choose 3 books out of 10 books to read during the summer. Order doesn't matter there either; you can read any book first.

Use the third when you need to figure out how many ways there are to select n objects with no repeats.

Example: How many ways can you seat 6 people in a row? You can put any 6 of them in the first chair but only 5 in the second (the 6th person has been placed in the first chair already), 4 in the third, etc.

 

[sacjumpman]

streetwolf i'd pay to have you tutor me QR 😉..... thanks for the input.

 

[Streetwolf]

I actually do tutor math 😉

 

[toothgut]

thank you SO much for clarifying that! I was having the hardest time understanding the difference too...
streetwolf, you rock!

 

[longlashes]

..

 

[Streetwolf]

Same equation.

m/n/r = m/(n*r)