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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...
Get it?
Now, you fill in the
girls and finish it off:
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