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.
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Case 1: One ring. There are 3 options for which ring you wear, and 5 options for which finger to put it on, so 5 × 3 .
Case 2a: Two rings, each on separate fingers. There are ( 2 3 ) ways to choose the two rings and P ( 5 , 2 ) = 5 × 4 ways to choose which fingers to put them on, so ( 2 3 ) × 5 × 4 .
Case 2b: Two rings, both on a single finger. There are ( 2 3 ) ways to choose the two rings, 5 ways to choose the finger, and 2 ! ways to stack the rings, so ( 2 3 ) × 5 × 2 ! .
Case 3a: Three rings, each on separate fingers. There are P ( 5 , 3 ) = 5 × 4 × 3 ways to put the rings on separate fingers.
Case 3b: Three rings, two are stacked and one is alone. There are ( 2 3 ) ways to choose the rings to be stacked, P ( 5 , 2 ) = 5 × 4 ways to choose the fingers (the stacked ring acts like a single ring), and 2 ! ways to arrange the stacked ring, so ( 2 3 ) × 5 × 4 × 2 !
Case 3c: Three rings, all stacked together. There are 5 options for which finger on which to place all three rings, and once you choose, there are 3 ! ways to stack the rings, so 5 × 3 !
Case 4: No rings. Only 1 way to do this.
Total: 5 × 3 + P ( 5 , 2 ) × ( 2 3 ) + 5 × ( 2 3 ) × 2 ! + P ( 5 , 3 ) + 5 × 4 × ( 2 3 ) × 2 ! + 5 × 3 ! + 1 = 3 1 6
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
Problem Loading...
Note Loading...
Set Loading...
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 = 1 5 ways.
For 2 rings, there are three ways to choose the two rings. If both rings are on different fingers, there are 5 ⋅ 4 = 2 0 ways. If both rings are on the same finger, there are 5 ⋅ 2 = 1 0 ways. This gives 3 ⋅ ( 2 0 + 1 0 ) = 9 0 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 = 6 0 , 3 ⋅ 5 ⋅ 4 ⋅ 2 = 1 2 0 , 5 ⋅ 6 = 3 0 ways, respectively, for a total of 6 0 + 1 2 0 + 3 0 = 2 1 0 ways in this case.
The total number of ways is 1 + 1 5 + 9 0 + 2 1 0 = 3 1 6 .