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The sum of all the digits used to write the whole numbers 10 through 13 is 1+0+1+1+1+2+1+3 = 10. What is the sum of all the digits used to write the whole numbers 1 through 110, inclusive?

 Jan 21, 2018
 #1
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i think you would just count up the 1's then the 2's and so on, then multiply and add.

 

so there is 1, eleven 1's in 10-19, eight 1's in 20-99, thirteen 1's in 100-110,

1+11+8+13 = 33

 

so there are 2, 12, and 102,  then eleven 2's in 20-29, seven 2's in 30-101.

(3*2) + (11*2) + (7*2) = 42

 

i see some patterns, but test it out by doing the 3's and 4's?

 

(4*3) + (11*3) + (6*3) =63  based on patterns

 

would the 4's = 84? test and see

 Jan 21, 2018
edited by Guest  Jan 22, 2018
 #8
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fyi, based on my patterns above, it equals 957 too

 

33+42+63+84+105+126+147+168+189 = 957

Guest Jan 22, 2018
edited by Guest  Jan 22, 2018
 #2
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0, 1, 2, 3..............106, 107, 108, 109, Will add first and last=0 +109 =10

Will add second to second last =1 + 108 =10........and so on.

Since we have: 110/2 =55 pairs, each equaling 10, we therefore have:

55 x 10 =550

The only number left is the 110 itself, and since it has a value of 2, we therefore have:

550 + 2 = 552. And that is what I think !!.

 Jan 21, 2018
 #3
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which one should i try?

SmartMathMan  Jan 21, 2018
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i only have two chances if i get it wrong 2 times ill have to start all over again with  new problems... tough work

SmartMathMan  Jan 21, 2018
 #5
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The method used by Guest #2 works ONLY for numbers all ending in 9s. Therefore, the following is the more accurate method:

 

0, 1, 2, 3..............96, 97, 98, 99. Will add first and last=0 +99 =18

Will add second to second last =1 + 98 =18........and so on.

Since we have: 100/2 =50 pairs, each equaling 18, we therefore have:

50 x 18 =900

The only numbers left 100 to 110 itself, and since they have:1+2+3+4+5+6+7+8+9+10+2=57

900 + 57 =957 - Sum of all digits from 1 to 110.

 Jan 21, 2018
 #6
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Here is an extremrly basic mockup of a number table from 0-99 for you to reference as I make the adding process easier.

 

 

I notice a few patterns here. The tens digit remains the same. Let's attempt to add the numbers 0-9.

 

0+1+2+3+4+5+6+7+8+9

 

The numbers enclosed by the underbraces sum to nine (9+0=9, 1+8=9, 2+7=9, 3+6=9. 4+5=9). There are 5 lots of this occurring with the numbers one to nine, so the sum of the tens digit in the first columns equals 95 or 45. This phenomenon occurs ten times or once per column, so let's multiply this by 10. 4510=450

 

Now, let's do the ones column. The ones column is different because each column increments the amount by 1. The sum of the ones column can be represented by the following sequence. 010+110+210+...+810+910. We can use algebra to group the common factor, so it simplifies to 10(1+2+3+...+8+9). Of course, we already know that the sum of the numbers 1-9 sums to 45 by our first calculation. 10 times that amount yields 450

 

Of course, we have only dealt with the sum of the digits from 0-99. However, if you think about it, the sum of 100-109 should be the same as adding the ones digit and the hundreds digits. Of course, this is a special case since the tens digits are all 0. We already know that 1+2+3+...+8+9=45. We know that the hundreds digit of the numbers 100-109 sums to 110=10. However, we have forgotten 110, which has a value of 2. 45+10+2=57 for the sum of the digits from 100-110. Now, let's add everything together.

 

450+450+57=957 or the sum of the digits from 0-110

 Jan 21, 2018
 #7
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Summed up on Wolfram/Alpha and it gives:

57 + sum_(b=0)^9( sum_(a=0)^9(a + b)) =957

https://www.wolframalpha.com/input/?i=57+%2B+%E2%88%91%5Ba%2Bb%5D,+a%3D0+to+9,+b%3D0+to+9

 Jan 22, 2018
 #9
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11  "1s"  in the hundreds digits  =  11

 

10 * 45  +  1  =  451  =  sum of  tens digits   

 

11 * 45  = 495  =  sum of units digits

 

 

So

 

11 +  10*45 + 1  +  11*45   =    957

 

 

cool cool cool

 Jan 22, 2018
 #10
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Here is a very simple method to add them all up.

 

Write each number as two digits as follows:

01, 02, 03..........to 97, 98, 98

Now, since we have 100 numbers each of which consists of 2 digits, we, therefore, have:

2 x 100 = 200 individual digits. The average value of each digit is: 1+2+3+4+5+6+7+8+9 =45/10 =4.5.

200 x 4.5 = 900 - this is the sum of all numbers from 1 to 100. And to this add the sum of all numbers from 100 to 110 inclusive =57.

So, the total is 900 + 57 = 957 - the sum of all numbers from 1 to 110.

 Jan 22, 2018

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