When will Ramon get the upgrade?Case 1: If he stays for 4 consecutive nights, or
Case 2: if he stays for 5 consecutive nights
Total # of outcomes:
From 8 days, how many sets of 5 days are possible? 8C5 = 56.
Favorable # of outcomes: Case 2: if he stays for 5 consecutive nights
Consider the 5 consecutive days as 1 block (X). Apart from X, we have 3 days among the 8 days (say O, O, O).
Number of ways Ramon could keep 5 of the 8 days consecutive --- is essentially --- all possible arrangements of X, O, O, O = 4!/3! = 4
We can visualize these easily ->
XOOO
OXOO
OOXO
OOOX
Where X is a block of 5 consecutive days.
Therefore,
# of favorable outcomes in Case 2 = 4 Case 1: If he stays for 4 consecutive nights
Consider the 4 consecutive days as 1 block (X). Consider the other day Ramon will have to make a visit as Y. Consider the other days as O,O,O.
We need to find the
number of arrangements of X,Y,O,O,O such that X and Y are never together (Because if X and Y are together, that would make it 5 consecutive days, not 4).
- # of arrangements of X, Y, O, O, O = 5!/3! = 20
- # of arrangements of X, Y, O, O, O where X and Y are together = 4!/3! x 2! = 8
Logic: consider XY as 1 block called P.
P, O, O, O can be arranged in 4!/3! ways. But we cannot forget that within the block P, we could have XY or YX i.e., 2! arrangements.
Hence, 4!/3! x 2! = 8
Therefore -
number of arrangements of X,Y,O,O,O such that X and Y are never together = 20 - 8 = 12
# of favorable outcomes in Case 1 = 12
# of favorable outcomes = 4 + 12 = 16
Hence,
Probability that he will be offered the upgrade:
16/56 =
2/7. Choice B.---
Harsha