FIND THE COOL, UNIQUE NO.

Number Theory Level pending

WHAT IS THE 1 ST 8 DIGIT NUMBER WHICH WHEN MULTIPLIED BY A MULTIPLE OF NINE LESSER THAN 99 WILL PRODUCE A SAME DIGIT 9 DIGIT NUMBER . FOR EXAMPLE 111111111


The answer is 12345679.

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1 solution

Akash Deep
Aug 4, 2014

l e t , 1.....1 ( 1 s 9 t i m e s ) = k = 99 ( 1122334 ) + 45 k = 9 ( 11 ( 1122334 ) + 5 ) k = 9 ( 12345674 + 5 ) k = 9 ( 12345679 ) l e t 12345679 = m n o w w e h a v e t o f i n d t h e s m a l l e s t 8 d i g i t n u m b e r w h i c h w h e n m u l t i p l i e d b y a t w o d i g i t n u m b e r l e s s t h a n 99 s o t h a t w e f i n d a 9 d i g i t n u m b e r w i t h a l l t h e d i g i t s s a m e . i t i s c l e a r t h a t k = 1....1 ( 1 s 9 t i m e s ) = 9 m a n d k 2 = 2.....2 ( 2 s 9 t i m e s ) 2 9 m = 2.....2 ( 2 s 9 t i m e s ) w e f i n d t h a t m i s t h e h e n c e r e q i r e d s m a l l e s t n u m b e r w h i c h w h e n m u l t i p l i e d b y a t w o d i g i t n u m b e r l e s s t h a n 99 s o t h a t w e f i n d a 9 d i g i t n u m b e r w i t h a l l t h e d i g i t s s a m e b e c a u s e w e g e t 18 a s a t w o d i g i t n u m b e r l e s s e r t h a n 99 a n d 12345679 a s t h e s m a l l e s t 8 d i g i t n u m b e r t o o b t a i n a 9 d i g i t n u m b e r w i t h a l l d i g i t s s a m e . let\quad ,1.....1(1's\quad 9\quad times)\quad =\quad k\quad =\quad 99*(1122334)\quad +\quad 45\\ k\quad =\quad 9\quad (11(1122334)\quad +\quad 5)\\ k\quad =\quad 9\quad (12345674\quad +\quad 5)\\ k\quad =\quad 9\quad (12345679)\quad let\quad 12345679\quad =\quad m\\ now\quad we\quad have\quad to\quad find\quad the\quad smallest\quad 8\quad digit\quad number\\ \quad which\quad when\quad multiplied\quad by\quad a\quad two\quad digit\quad number\quad \\ less\quad than\quad 99\quad so\quad that\quad we\quad find\quad a\quad \\ 9\quad digit\quad number\quad with\quad all\quad the\quad digits\quad same.\\ it\quad is\quad clear\quad that\quad k\quad =\quad 1....1(1's\quad 9\quad times)\quad =\quad 9m\\ and\quad k\quad *\quad 2\quad =\quad 2.....2(2's\quad 9\quad times)\\ \quad \quad 2*9*\quad m\quad =\quad \quad 2.....2(2's\quad 9\quad times)\\ we\quad find\quad that\quad m\quad is\quad the\quad hence\quad reqired\quad smallest\quad \\ number\quad which\quad whenmultiplied\quad by\quad a\quad two\quad digit\quad \\ number\quad less\quad than\quad 99\quad so\quad that\quad we\quad find\quad a\quad \\ 9\quad digit\quad number\quad with\quad all\quad the\quad digits\quad same\quad because\\ \quad we\quad get\quad 18\quad as\quad atwo\quad digit\quad number\quad lesser\quad than\quad 99\\ \quad and\quad 12345679\quad as\quad the\quad smallest\quad 8\quad digit\quad \\ number\quad to\quad obtain\quad a\quad 9\quad digit\quad number\quad with\quad all\quad digits\quad same.\\ \\

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