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Given: Messi plans a trip to Qatar World Cup 2022 and intends to invite at most 9 of his 10 friends with him.
Asked: If half of his 10 friends are football players and Messi wants to invite at least 4 football players to Qatar, then how many different groups of friends can Messi form for the trip ?

Case 1: Group of friends for the trip include 4 football players
Number of ways to select 4 football players out of 5 = 5C4 = 5
Number of ways to select at most 5 other friends = 5C0 + 5C1 + 5C2 + 5C3 + 5C4 + 5C5 = 1 + 5 + 10 + 10 + 5 + 1 = 32
Total number of ways to select friends for the trip including 4 football players = 5*32 = 160

Case 2: Group of friends for the trip include all 5 football players
Number of ways to select 5 football players out of 5 = 5C5 = 1
Number of ways to select at most 4 other friends = 5C0 + 5C1 + 5C2 + 5C3 + 5C4 = 1 + 5 + 10 + 10 + 5 = 31
Total number of ways to select friends for the trip including 5 football players = 31

Total number of group of friends for the trip = 160 + 31 = 191

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Quote:
Messi plans a trip to Qatar World Cup 2022 and intends to invite at most 9 of his 10 friends with him. If half of his 10 friends are football players and Messi wants to invite at least 4 football players to Qatar, then how many different groups of friends can Messi form for the trip ?


A. 63
B. 64
C. 191
D. 192
E. 1023

I solved this question using case by case approach. If anyone has a shorter approach, please feel free to tag me.

Assuming that Messi will take 9 friends (all 5 footballers): 5C5*5C4 = 5
Assuming that Messi will take 9 friends (but 4 footballers): 5C4*5C5 = 5
Therefore for 9 friends, the number of teams possible = 5+5 = 10

If you calculate this up to 4 friends (since he will take at least 4 footballer friends), the number of possible selections is 191.
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We know that at max Messi can invite 9 of his friends, however he wants to invite at least 4 football players. so the number of players that can be invited will lie between 4 to 9, both inclusive

Number of ways Messi can invite 4 players = 5C4 = 5
Number of ways Messi can invite 5 players = 5C4*5C1 + 5C5 = 26
Number of ways Messi can invite 6 players = 5C4*5C2 + 5C5*5C1 = 55
Number of ways Messi can invite 7 players = 5C4*5C3 + 5C5*5C2 = 60
Number of ways Messi can invite 8 players = 5C4*5C4 + 5C5*5C3 = 35
Number of ways Messi can invite 9 players = 5C4*5C5 + 5C5*5C4 = 10

If we sum up the above we obtain a total of 191

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