Questions   
Sort: 
 #1
avatar
+1

There are four ways in which a mailbox could have exactly 3 letters: the first mailbox could have 3 letters, the second mailbox could have 3 letters, the third mailbox could have 3 letters, or the fourth mailbox could have 3 letters.

Let's consider each case separately:

Case 1: The first mailbox has 3 letters.

In this case, there are 3 letters left to be placed in the remaining 3 mailboxes. Each of the remaining 3 letters can be placed in any of the 3 remaining mailboxes, so there are 3 choices for each of the 3 remaining letters. Therefore, there are 3^3 = 27 ways in which the remaining letters could be placed in the remaining mailboxes.

Case 2: The second mailbox has 3 letters.

This case is similar to Case 1, but we need to account for the fact that the 3 letters that go in the second mailbox could be any 3 of the 4 letters. There are 4 ways to choose which 3 letters go in the second mailbox, and then there are 3 choices for the remaining letter, and 2 choices for where to put it. Therefore, there are 4 x 3 x 2 = 24 ways in which the letters could be placed in this case.

Case 3: The third mailbox has 3 letters.

This case is also similar to Case 1, but we need to account for the fact that the 3 letters that go in the third mailbox could be any 3 of the 4 letters. There are 4 ways to choose which 3 letters go in the third mailbox, and then there are 3 choices for the remaining letter, and 2 choices for where to put it. Therefore, there are 4 x 3 x 2 = 24 ways in which the letters could be placed in this case.

Case 4: The fourth mailbox has 3 letters.

This case is similar to Case 1, but we need to account for the fact that the 3 letters that go in the fourth mailbox could be any 3 of the 4 letters. There are 4 ways to choose which 3 letters go in the fourth mailbox, and then there are 3 choices for the remaining letter, and only 1 choice for where to put it. Therefore, there are 4 x 3 x 1 = 12 ways in which the letters could be placed in this case.

Therefore, the total number of ways in which one of the mailboxes could have exactly 3 letters is 27 + 24 + 24 + 12 = 87.

The total number of ways in which the 4 letters could be placed in the 4 mailboxes is 4^4 = 256.

Therefore, the probability that one of the mailboxes has exactly 3 letters is 87/256.

Mar 20, 2023
Mar 19, 2023
 #1
avatar
0

We can break down the problem into two cases based on the value of x:

Case 1: x is 0 or 1.

If x is 0 or 1, then there are three possibilities for the value of y: 0, 1, or 2. For each of these values of y, we can break down the problem further based on the value of z:

If z is 0, then we need to find the number of solutions to the equation u+v+w = -x-y, where u, v, and w are nonnegative integers. This is a standard stars and bars problem, and the number of solutions is (3+2-1) choose 2 = 6 choose 2 = 15.

If z is 1, then we need to find the number of solutions to the equation u+v+w = -x-y+z-1. Again, this is a stars and bars problem, and the number of solutions is (3+2-1) choose 2 = 15.

If z is 2, then we need to find the number of solutions to the equation u+v+w = -x-y+z-2. Once again, this is a stars and bars problem, and the number of solutions is (3+2-1) choose 2 = 15.

Therefore, in this case, the total number of solutions is 3*(15+15+15) = 135.

Case 2: x is 2.

If x is 2, then y must be 0. We can break down the problem further based on the value of z:

If z is 0, then we need to find the number of solutions to the equation u+v+w = -x-y = -2, where u, v, and w are nonnegative integers. This is a stars and bars problem, and the number of solutions is (3+2-1) choose 2 = 6 choose 2 = 15.

If z is 1, then we need to find the number of solutions to the equation u+v+w = -x-y+z-1 = -1. This is also a stars and bars problem, and the number of solutions is (3+2-1) choose 2 = 15.

If z is 2, then we need to find the number of solutions to the equation u+v+w = -x-y+z-2 = 0. This is once again a stars and bars problem, and the number of solutions is (3+2-1) choose 2 = 15.

Therefore, in this case, the total number of solutions is 15+15+15 = 45.

Adding up the number of solutions from both cases, we get a total of 135+45 = 180 solutions to the equation.

Mar 19, 2023

0 Online Users