way to arrangement of number from 9 digits from 1 to 9

Question

I want to take 6 digits from 9 digits and want to form number . 9 digits include from 1 to 9 . In these 6 digits , the digits can be in any number . In these 6 digits , one feature must be included : last 3 digits can not contain digits from first 3 space . What is the no of way to form numbers of such number ?

Any kind of advice will be of great help .

Answer 1

HINT

Break this up into $3$ cases:

A. Numbers where the first three digits are all the same

B. Numbers where the first three digits are all different

C. Numbers where two of the first three digits are the same but the third is different