A,B,C and D have mixed up their respective secret codes in an envelope in How many ways can they pick up their codes in such a manner that no one gets his own code ?

Explanation

To solve the problem of how many ways A, B, C, and D can pick up their secret codes such that no one gets their own code, we will use the concept of derangements. A derangement is a permutation of a set where none of the elements appear in their original position.

### Step-by-Step Solution:

1. **Understanding the Problem**:
We need to find the number of ways to arrange the codes of A, B, C, and D such that none of them receives their own code. This is a classic derangement problem.

2. **Derangement Formula**:
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