Number __GAME__!!!!!!

How many Natural Numbers not exceeding 4321 can be formed with the Digits 1, 2, 3, and 4 when the digits can repeat?

This is a part of Classical Combinatorics


The answer is 313.

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4 solutions

We have to consider the total numbers N N by stages as follows:

U p t o N u m b e r s 4 4 = 4 44 4 × 4 = 16 444 4 × 4 × 4 = 64 3444 3 × 4 × 4 × 4 = 192 4244 1 × 2 × 4 × 4 = 32 4314 1 × 1 × 1 × 4 = 4 4321 1 × 1 × 1 × 1 = 1 \begin{matrix} Up \space to & & Numbers \\ 4 & 4= & 4 \\ 44 & 4\times 4= & 16 \\ 444 & 4 \times 4\times 4= & 64 \\ 3444 & 3 \times 4 \times 4\times 4= & 192 \\ 4244 & 1 \times 2 \times 4\times 4= & 32 \\ 4314 & 1 \times 1 \times 1\times 4= & 4 \\ 4321 & 1 \times 1 \times 1\times 1= & 1 \end{matrix}

Therefore, the total number of natural numbers formed with digits 1, 2, 3 and 4 with repetitions N = 4 + 16 + 64 + 192 + 32 + 4 + 1 = 313 N = 4+16+64+192+32+4+1 = \boxed{313}

I used the EXACT SAME WAY

Vaibhav Prasad - 6 years, 3 months ago

I did it the same way. The answer reminded me of 8 mile :)

Aditya Mishra - 6 years, 3 months ago

I Did it the same way! Btw nice solution @Chew-Seong Cheong +1

Harsh Shrivastava - 6 years, 3 months ago
Adarsh Kumar
Sep 5, 2014

T h e q u e s t i o n s a y s t h a t w e h a v e t o f i n d n a t u r a l n u m b e r s u s i n g d i g i t s f r o m 1 t o 4. The\ question\ says\ that\ we\ have\ to\ find\ natural\ numbers\ using\ digits\ from\ 1\ to\ 4. B u t i t h a s n t s p e c i f i e d o n t h e n u m b e r o f d i g i t s o f e a c h n u m b e r . But\ it\ hasn't\ specified\ on\ the\ number\ of\ digits\ of\ each\ number. T h u s w e h a v e t o f i n d a l l n u m b e r s w i t h d i g i t s f r o m 1 t o 4 n o t e x c e e d i n g 4321 / Thus\ we\ have\ to\ find\ all\ numbers\ with\ digits\ from\ 1\ to\ 4 \ not\ exceeding\ 4321/ W e w i l l d o t h i s c a s e w i s e . We\ will\ do\ this\ casewise. C a s e 1 : Case\ 1: S i n g l e d i g i t n u m b e r s Single\ digit\ numbers O b v i o u s l y 4 c a n b e f o r m e d . Obviously\ 4\ can\ be\ formed. c a s e 2 : case\ 2: T w o d i g i t n u m b e r s Two\ digit\ numbers W e h a v e f o u r p o s s i b i l i t i e s f o r t h e u n i t d i g i t a n d f o u r p o s s i b i l i t i e s f o r t h e t e n s d i g i t We\ have\ four\ possibilities\ for\ the\ unit\ digit\ and\ four\ possibilities\ for\ the\ tens\ digit T h u s , 16 n u m b e r s c a n b e f o r m e d . Thus,\ 16\ numbers\ can\ be\ formed. C a s e 3 : Case\ 3: 3 d i g i t n u m b e r s 3\ digit\ numbers W e h a v e f o u r p o s s i b i l i t i e s f o r e a c h p l a c e t h u s , 64 n u m b e r s a r e p o s s i b l e . We\ have\ four\ possibilities\ for\ each\ place\ thus,\ 64\ numbers\ are\ possible. T i l l n o w i t h a s b e e n p r e t t y s t r a i g h t f o r w a r d b u t t h e n e x t c a s e h a s s o m e c o m p l i c a t i o n s a s w e h a v e a r e s t r i c t i o n . Till\ now\ it\ has\ been\ pretty\ straight\ forward\ but\ the\ next\ case\ has\ some\ complications\ as\ we\ have\ a\ restriction. C a s e 4 : Case\ 4: F o u r d i g i t n u m b e r s Four\ digit\ numbers S u b c a s e 1 : Subcase\ 1: 1 i s t h e d i g i t i n t h e t h o u s a n d t h s p l a c e 1\ is\ the\ digit\ in\ the\ thousandths\ place T h e n w e h a v e 4 p o s s i b i l i t i e s f o r e a c h p l a c e , a s a n u m b e r w h i c h h a s 1 i n i t s t h o u s a n d t h s p l a c e w i l l b e l e s s t h a n 4321 Then\ we\ have\ 4\ possibilities\ for\ each\ place,\ as\ a\ number\ which\ has\ 1\ in\ its\ thousandths\ place\ will\ be\ less\ than\ 4321 T h u s , w e h a v e 64 n u m b e r s . Thus,\ we\ have\ 64\ numbers. S u b c a s e 2 : Subcase\ 2: 2 i s t h e d i g i t i n t h e t h o u s a n d t h s p l a c e 2\ is\ the\ digit\ in\ the\ thousandths\ place T h e n w e h a v e 4 p o s s i b i l i t i e s f o r e a c h p l a c e , a s a n u m b e r w h i c h h a s 2 i n i t s t h o u s a n d t h s p l a c e w i l l b e l e s s t h a n 4321 Then\ we\ have\ 4\ possibilities\ for\ each\ place,\ as\ a\ number\ which\ has\ 2\ in\ its\ thousandths\ place\ will\ be\ less\ than\ 4321 T h u s , w e h a v e 64 n u m b e r s Thus,\ we\ have\ 64\ numbers S u b c a s e 3 : Subcase\ 3: 3 i s t h e d i g i t i n t h e t h o u s a n d t h s p l a c e . 3\ is\ the\ digit\ in\ the\ thousandths\ place. T h e n w e h a v e 4 p o s s i b i l i t i e s f o r e a c h p l a c e , a s a n u m b e r w h i c h h a s 3 i n i t s t h o u s a n d t h s p l a c e w i l l b e l e s s t h a n 4321 Then\ we\ have\ 4\ possibilities\ for\ each\ place,\ as\ a\ number\ which\ has\ 3\ in\ its\ thousandths\ place\ will\ be\ less\ than\ 4321 S u b c a s e 4 : Subcase\ 4: 4 i s t h e d i g i t i n t h e t h o u s a n d t h s p l a c e . 4\ is\ the\ digit\ in\ the\ thousandths\ place. I f w e h a v e 1 o r 2 i n t h e h u n d r e t h s p l a c e t h e n 16 + 16 n u m b e r s If\ we\ have\ 1\ or\ 2\ in\ the\ hundreths\ place\ then\ 16\ +\ 16\ numbers I f t h e n u m b e r h a s 3 i n t h e h u n d r e t h s p l a c e a n d 1 i n t h e t e n s p l a c e t h e n t h e r e a r e 4 p o s s i b i l i t i e s . If\ the\ number\ has\ 3\ in\ the\ hundreths\ place\ and\ 1\ in\ the\ tens\ place\ then\ there\ are\ 4\ possibilities. I f t h e n u m b e r h a s 3 i n t h e h u n d r t e t h s p l a c e a n d 2 i n t h e t e n t h s p l a c e t h e n t h e r e i s o n l y o n e p o s s i b i l i t y . If\ the\ number\ has\ 3\ in\ the\ hundrteths\ place\ and\ 2\ in\ the tenths\ place\ then\ there\ is\ only\ one\ possibility. A d d i n g g i v e s 313. Adding\ gives\ 313.

Lavneesh Nyol
Oct 19, 2014

As the maximum number which can be formed is 4444. Hence, we can easily calculate the numbers from 4322 upto 4444 formed with the Digits 1,2,3 & 4 only, which comes out to be 27. Now we can calculate the total numbers which can be made using the digits (1,2,3 &4) which is 4^4(4 digit numbers) + 4^3 (3 digit numbers) +4^2 (2 digit numbers) +4 (single digit number) which comes out to be 340.

Hence the answer is 340-27= 313 \boxed{313}

I I a l s o also d i d did i t it t h e the s a m e w a y same way

Parth Lohomi - 6 years, 7 months ago
Edward Jiang
Aug 30, 2014

PARI/GP code:

is_ok(n)=m=Vec(Str(n));i=1;while(i<=#m,if(eval(m[i])==(1||2||3||4),i++,return(0);break));return(1)
answer=0;for(i=1,4321,if(is_ok(n),answer++));return(answer)

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