In how many ways can gold, silver and bronze medals be awarded among 10 athletes?
Medals differ: 10 × 9 × 8 = 720
Trap: 120 ignores which medal is which.
Answer: 720
Permutations and Combinations · Exercise 4 of 5
65 moderate and difficult permutations and combinations questions, with points on a circle to read, in the style of IBPS / SBI PO and Clerk, RRB NTPC and Group D, UPSC CSAT, state PSC and IT campus placement tests.
In how many ways can gold, silver and bronze medals be awarded among 10 athletes?
Medals differ: 10 × 9 × 8 = 720
Trap: 120 ignores which medal is which.
Answer: 720
A president and a secretary are chosen from 10 members, but member A will not serve as secretary. In how many ways?
All: 10 × 9 = 90
A as secretary: 9 choices of president
90 − 9 = 81
Answer: 81
A polygon has three times as many diagonals as sides. How many sides does it have?
n(n − 3)/2 = 3n → n − 3 = 6 → n = 9
Answer: 9
How many triangles can be formed by joining the vertices of a 12-sided polygon?
Any 3 vertices: 12C3 = 220
Trap: 1,320 counts order.
Answer: 220
At a party of 15 people (6 married couples and 3 single people), everyone shakes hands with everyone except their own spouse. How many handshakes are there?
All pairs: 15C2 = 105
Minus 6 spouse pairs = 99
Answer: 99
In a league of 12 teams, every team plays every other team twice (home and away). How many matches are played?
Pairs: 12C2 = 66; each pair plays twice
66 × 2 = 132
Trap: 66 counts each pair once.
Answer: 132
A knockout tournament starts with 32 teams. How many matches are played in all?
Each match knocks out one team; 31 must go out
31 matches
Trap: 496 = 32C2 is a round robin.
Answer: 31
In how many ways can 6 different books be given to 3 students so that each gets 2?
6C2 × 4C2 × 2C2 = 15 × 6 × 1 = 90
Trap: 15 makes unnamed bundles.
Answer: 90
In how many ways can 6 different books be tied into 3 bundles of 2 books each?
Named bundles: 90
Bundles are alike → ÷ 3! = 15
Trap: 90 treats the bundles as labelled.
Answer: 15
In how many ways can 3 letters be dropped into 5 post boxes?
Each letter chooses one of 5 boxes: 5³ = 125
Trap: 243 = 3⁵ reverses the roles.
Answer: 125
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