Question 1132815
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These 3-digit numbers are such that they


    - <U>EITHER</U> contain at least two digits 5,        (category 1)

    - <U>OR</U>     contain both the digits  5 and 0,     (category 2)

    - <U>OR</U>     contain two digits 0.                 (category 3).


In <U>category 1</U>  we have 13 numbers

    155, 255, 355, 455, 555,
    515, 525, 535, 545, 
    551, 552, 553, 554.


In <U>category 2</U> we have 18 numbers

    105, 205, 305, 405, 505,
    150, 250, 350, 450, 550,
    510, 520, 530, 540,
    501, 502, 503, 504.


In <U>category 3</U> we have 5 numbers.

    100, 200, 300, 400, 500.


In all, there are  13 + 18 + 5 = 36 such numbers.


It may happen that I missed something, but the idea is clear.


This hint is  <U>EITHER</U>  the full solution  <U>OR</U>  a good push for those who study at RSM (Russian School of Mathematics).



P.S.  If you think that the numbers of <U>category 3</U>  should not be included to the set of R-numbers  

      (in this part your definition is not certain), then exclude them . . . 
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