A person has 5 tickets for a concert and she wants to invite 4 of her 8 best friends. How many choices does she have, if two of her friends do not get along and cannot be both invited?

55

Step-by-step explanation:

Combinations formula is used to make choice of 'R' out of 'N' options =

N(C)R = N ! / [ R ! . (N-R)! ]

Total choices to choose 4 out of 8 friends = 8C4

= 8! / (4! 4!)  

= 70

Choices for calling them 2 together = 2C2 x 6C2

= 1 x [ 6! / (2! 4!)]

= 15

So : Number of choices that the 2 friends are not called together = Total choices - choices they are called together

= 70 - 15 = 55

$53.50

step-by-step explanation:

you have to subtract the amount of money chuck earned ($72.50) by the cost of the gas for the mower ($11.25) and the new string for the trimmer he bought ($7.75).

72.50-11.25=61.25

and then subtract $61.25 by the second thing he bought (the new string for the trimmer)

61.25-7.75=53.5

so in total it's $53.50 (53 dollars and 50 cents)

it would take 7/10 of an hour too fill the pool.

step-by-step explanation: