Too many wedding rings.

A person has three distinguishable rings numbered 1 through 3. How many ways can he put his 3 rings on his five fingers?

Clarification 1: He can choose to wear any number of rings (even 0).
Clarification 2: The order of rings on each finger matters from top to bottom.

201 316 801 771

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3 solutions

Jon Haussmann
Nov 27, 2015

For 0 rings, there is one way.

For 1 ring, there are three ways to choose the ring, then five ways to choose the finger, so 3 5 = 15 3 \cdot 5 = 15 ways.

For 2 rings, there are three ways to choose the two rings. If both rings are on different fingers, there are 5 4 = 20 5 \cdot 4 = 20 ways. If both rings are on the same finger, there are 5 2 = 10 5 \cdot 2 = 10 ways. This gives 3 ( 20 + 10 ) = 90 3 \cdot (20 + 10) = 90 ways in this case.

For 3 rings, either (i) all rings are on different fingers, (ii) two rings are on one finger and one ring is on a different finger, or (iii) all rings are on the same finger. There are 5 4 3 = 60 5 \cdot 4 \cdot 3 = 60 , 3 5 4 2 = 120 3 \cdot 5 \cdot 4 \cdot 2 = 120 , 5 6 = 30 5 \cdot 6 = 30 ways, respectively, for a total of 60 + 120 + 30 = 210 60 + 120 + 30 = 210 ways in this case.

The total number of ways is 1 + 15 + 90 + 210 = 316 1 + 15 + 90 + 210 = 316 .

Ariella Lee
Dec 16, 2015

Case 1: One ring. There are 3 3 options for which ring you wear, and 5 5 options for which finger to put it on, so 5 × 3 5\times3 .

Case 2a: Two rings, each on separate fingers. There are ( 3 2 ) \binom{3}{2} ways to choose the two rings and P ( 5 , 2 ) = 5 × 4 P(5,2)=5\times4 ways to choose which fingers to put them on, so ( 3 2 ) × 5 × 4 \binom{3}{2}\times5\times4 .

Case 2b: Two rings, both on a single finger. There are ( 3 2 ) \binom{3}{2} ways to choose the two rings, 5 5 ways to choose the finger, and 2 ! 2! ways to stack the rings, so ( 3 2 ) × 5 × 2 ! \binom{3}{2}\times5\times2! .

Case 3a: Three rings, each on separate fingers. There are P ( 5 , 3 ) = 5 × 4 × 3 P(5,3)=5\times4\times3 ways to put the rings on separate fingers.

Case 3b: Three rings, two are stacked and one is alone. There are ( 3 2 ) \binom{3}{2} ways to choose the rings to be stacked, P ( 5 , 2 ) = 5 × 4 P(5,2)=5\times4 ways to choose the fingers (the stacked ring acts like a single ring), and 2 ! 2! ways to arrange the stacked ring, so ( 3 2 ) × 5 × 4 × 2 ! \binom{3}{2}\times5\times4\times2!

Case 3c: Three rings, all stacked together. There are 5 5 options for which finger on which to place all three rings, and once you choose, there are 3 ! 3! ways to stack the rings, so 5 × 3 ! 5\times3!

Case 4: No rings. Only 1 1 way to do this.

Total: 5 × 3 + P ( 5 , 2 ) × ( 3 2 ) + 5 × ( 3 2 ) × 2 ! + P ( 5 , 3 ) + 5 × 4 × ( 3 2 ) × 2 ! + 5 × 3 ! + 1 = 316 5\times3+P(5,2)\times\binom{3}{2}+5\times\binom{3}{2}\times2!+P(5,3)+5\times4\times\binom{3}{2}\times2!+5\times3!+1=\boxed{316}

Phi Li
Nov 20, 2015

Cases will be based on the number of rings used.

Case 1 (No rings): 1
Case 2 (1 ring): 5 * 4
Case 3 (2 rings):
Placement 1 (2 rings on same finger): 5 * (4 choose 2) * 2!
Placement 2 (2 rings on different fingers): (5 choose 2) * 4 * 3
Case 4 (3 rings): * Here comes the hard part Placement 1 (one ring on each finger): (5 choose 3) * 4 * 3 * 2
Placement 2 (two rings on one finger and one ring on the other): 5 * 4 * (4 choose 2) * 2!
Placement 3 (three rings on the same finger): 5 * (4 choose 3) * 3!



1 + 20 + 60 + 120 + 240 + 240 + 120 = 801 ways

Can you explain why 1 ring is 5 × 4 5 \times 4 ? Shouldn't it be 5 × 3 5 \times 3 ?

Calvin Lin Staff - 5 years, 6 months ago

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