Hi guys: Can any one please expain this question according to that answer ? Thanks.

A 5 digit number is formed according to the following rules:

The number contains only the digits 1,2,3,4 and these digits may be repeated or unused.

The first digit is 1.

If D1D2 are consecutive digits and D1 not equals to 4, then D1 < D2

If D1D2 are consecutive digits and D1 = 4, then D2 = 1.

(In other words, digits must be in increasing order, except that any 4 must followed by 1. For example, 12413 is one such number.) How many numbers of this type are there? Use a decision tree to compute your answer.

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