# Probability: No. of 4-letter string formed by ...

• Mar 8th 2010, 07:31 AM
pozat
Probability: No. of 4-letter string formed by ...
Question

How many no. of 4-letter strings can be formed by the letters {A,B,C,D,E,F} with no repetitions and A cannot be the first letter?

(Thinking)

I know the total no. of 4-letter strings with no repetitions is 6x5x4x3=360

Then it comes to the next consideration: how many no. of combination with A being the first letter...

Should it be 5x4x3=60?[/SIZE]

Not sure if this is correct ...
• Mar 8th 2010, 07:51 AM
Plato
Quote:

Originally Posted by pozat
Question

How many no. of 4-letter strings can be formed by the letters {A,B,C,D,E,F} with no repetitions and A cannot be the first letter?

(Thinking)

I know the total no. of 4-letter strings with no repetitions is 6x5x4x3=360

Then it comes to the next consideration: how many no. of combination with A being the first letter...

Should it be 5x4x3=60?[/SIZE]

That is correct. It is also $\displaystyle 5\cdot 5\cdot 4\cdot 3=300$