Here are some notations for this thing...
We use "P" for "permutation":
Look for one of these on your calculator... Now, try your calculator with the last two problems.
(You usually put the n guy in first, then hit the button, then put the r guy in and hit = .)
* Again, it's better
to figure these guys out with slots than to just try to
memorize a formula!
Why? Because sometimes you have to think... and you need the practice!
Check it out:
You have 9 boys and 12 girls. You're going to take 4 boys and 3 girls and arrange them in a line boy - girl - boy - girl. How many ways can you do it?
Let's do the slot thing:
Let's do the boys first...
Now, you fill in the girls and finish it off: