Think of letters  $ABC$ , how many possible ways to rearrange this three letters if each letter can only be used once? (So no AAA or BBB) 

Think of letters  $AB$  , how many different ways to rearrange these two letters if each letter is reusable? (So allow AA or BB)

Think of letters  $ABC$ , how many different ways to rearrange these 3 letters if each letter is reusable? (So allow AAA or BBB or CCC)

So, now think of letters  $T\ H\ I\ N\ K$ , how do we find how many different ways to rearrange those letters if each letter can only be used once? 

Think of letters  $A\ M\ O\ N\ G\ U\ S$ , how many different ways to rearrange these letters if each letter is reusable?

Now get your scientific calculator. Can you find ! symbol?

So, now let's find out what is the value of  $5!=$ 

Once again, what is the value of  $6!-3!=$ 

Is  $5!$ the same with  $\left(5\times4\times3\times2\times1\right)$  ?

Go get your scientific calculator again. Can you find nPr?

Okay, excellent! Now, what is the value of  $5P3$ ?

Again!  $10P7$ =

 $5P9=$  

One more time... Can you find nCr on your scientific calculator?

Now let's try it... What is the value of  $5C2=$ 

Again! What is the value of  $8C10=$ 

are  $6!$ and  $6P6$  the same?

 $3!\times3P2\times3C1=$  

Now I know how to...