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what is the probablitty the fourth photograph will be of katie?

Robert has an envelope containing 24 photographs. Six of the photographs are of Robert, and the rest are of his friend Katie. He will choose 4 photographs at random. If the first 3 are of Robert.
 
a. 1/2
b.3/4
c. 6/7
d. 9/10
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2 Answers

Like Phillip said,
 
# of Photos of Katie = PK = 18
# of Photos of Robert = PR = 6
Total # of photos = 18 +6 = 24
 
After getting 3 photographs of Robert (24 -3),
 
PK = 18
PR = 3
TOTAL # of Photos remaining = 18 + 3 = 21 photos
 
=(18=number of photos of Katie)/(21=number of remaining photos) = 6/7 the answer is C.
Again, after removing 3 pictures of Robert, there are 21 photos remaining in the envelope, 3 of Robert and 18 of Katie.  The probability that the next photo will be of Katie is:
 
P = (Number of Photos of Katie Remaining) / (Total Number of Photos Remaining)
 
Remember to reduce the fraction. Let me know what you get!