System
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## 4-digit number

How many 4-digit numbers are there with no digit repeated?
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## Triangular matrix

If U1, U2 are upper triangular and L1, L2 are lower triangular, which of the following are triangular?  U1 + U2  U1U2  U12  U1 + L1  U1L1  L1 +...
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## Probability of Positive Integers being Coprime !

What is the probability that six randomly chosen positive integers are relatively prime ?