amy is choosing a 2-letter password from the letters a, b, c, d, e, and f. the password cannot have the same letter repeated in it. how many such passwords are possible?

Amy can choose from 60 possible passwords.

Solution:

Given that the two-letter password is from the letters A, B, C, D, E and F

Total number of letters = 6  

The password is a two digit letter and repetition of letters is not allowed.

Method 1:

\therefore The number of ways of obtaining an ordered subset of r elements from a set of n elements is given by

So, the total possible password = = =

On cancelling the in numerator and denominator we get,

Here we get, = 30  possible passwords

Method 2:

There are 6 possible choices for the first term of the password and 5 choices for the second term (i.e. leaving the selected letter of the term)

So,

step-by-step explanation:

12: 00 p.m. to 7: 00 p.m. is a total of 7 hours.

4: 00 p.m. to 7: 00 p.m. is a total of 3 hours.

this means that from 4: 00 p.m to 7: 00 p.m. this represents 3/7 of what was lost from 12: 00 p.m. to 7: 00 p.m.

after knowing this, after doing 11,600 - 6,800 = 4,800

4,800 represents 3/7 of what was lost for 7 hours.

we then divide 4,800 by 3 which gets 1,600.

because we know that 1,600 represents 1/7 of how much water was lost, we just have to multiply by 7, which gets 11,200.   after adding 11,200 to how much water was left at 7: 00 p.m. which was 6,800. 11,200 + 6,800 = 18,000

this means that the pool originally had 18,000 gallons in the pool.

you can also check your work by doing 18,000 - 4(1600), which becomes           1800 - 6400 which gets you 11,600.