Muhammad's skipping list

Eva lists the positive integers, but skips all numbers containing a digit divisible by 3 3 . For example, the first ten numbers she lists are 1 , 2 , 4 , 5 , 7 , 8 , 11 , 12 , 14 , 15 1, 2, 4, 5, 7, 8, 11, 12, 14, 15 . What is the 10 0 th 100^{\text{th}} number in her list?

This problem is shared by Muhammad A.

Details and assumptions

The number 13 13 is not included in the list, since it contains the digit 3, which is divisible by 3.


The answer is 255.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

7 solutions

Jessica Jaba
Jul 23, 2013

100=244 (base 6) See that if x is in the yth place in the base 6 representation of n, then the yth place of nth number that is in Eva's list is the xth possible digit (in ascending order of 1,2,4,5,7,8). The 2nd digit that can be used is 2. Therefore, the number in the hundreds place is 2. The 4th digit that can be used is 5. Therefore, the numbers in the tens and ones places are both 5. The 100th number in Eva's list is 255.

Gilbert Chia
Jul 23, 2013

From 1-100, there are only 42 which will fit into the list. 1-10 has 6, 11-20 has 6, 21-30 has 6, 41-50 has 6, 71-80 has 6, 81-90 has 6, so we have a total of 42 integers from 1-100 that will fit into the list. For 100-200 however, there is 1 less pair as 101-110 doesn't count into the list. So for 100-200 we have 36 integers that will fit into the list, and now we still need 100-42-30 = 22 more integers that fit in the list. Since 211-220 has another 6, 221-230 another 6, 241-250 another 6, and we still need 22-6-6-6=4 more fitting integers. From there, we count that the 4th integer we need is 251,252,254,255. So 255 is the 42+36+6+6+6+4= 100th integer.

The numbers she wrote don't contain 0,3,6 or 9.So the digits of every number she wrote are from the other six digits.So the total numbers she wrote from 1 to 99 is 6*6=36.The numbers she wrote from 100 to 199 is also 36.Because the first digit of these numbers is 1 which is not divisible by 3.By the same reason the 100th number of the list is 200+(28th number of the list). And the 28th number is 55.So the answer is 255.

Victor Carnaúba
Jul 26, 2013

number by number I did kkk

same lol ...............................

math man - 6 years, 10 months ago
Arslan Mushtaq
Jul 26, 2013

We lose the digits 0, 3, 6, and 9. We have six single digit numbers, and 36 two digit number. The 100th number is the 58th three digit number. 58 = 1 6^2 + 3 6^1 + 4*6^0 So that is the 2nd numeral, the 4th numeral and the 4th numeral Which is 255.

Enumerating 1st to 100th term: 1, 2, 4, 5, 7, 8, 11, 12, 14, 15, 17, 18, 21, 22, 24, 25, 27, 28, 41, 42, 44, 45, 47, 48, 51, 52, 54, 55, 57, 58, 71, 72, 74, 75, 77, 78, 81, 82, 84, 85,

Rifat Khan
Jul 23, 2013

We lose the digits 0, 3, 6, and 9. We have six single digit numbers, and 36 two digit number. The 100th number is the 58th three digit number. 58 = 1 6^2 + 3 6^1 + 4*6^0 So that is the 2nd numeral, the 4th numeral and the 4th numeral Which is 255.

Enumerating 1st to 100th term: 1, 2, 4, 5, 7, 8, 11, 12, 14, 15, 17, 18, 21, 22, 24, 25, 27, 28, 41, 42, 44, 45, 47, 48, 51, 52, 54, 55, 57, 58, 71, 72, 74, 75, 77, 78, 81, 82, 84, 85, 87, 88, 111, 112, 114, 115, 117, 118, 121, 122, 124, 125, 127, 128, 141, 142, 144, 145, 147, 148, 151, 152, 154, 155, 157, 158, 171, 172, 174, 175, 177, 178, 181, 182, 184, 185, 187, 188, 211, 212, 214, 215, 217, 218, 221, 222, 224, 225, 227, 228, 241, 242, 244, 245, 247, 248, 251, 252, 254, 255 So the 100th Number of the list is 255.

but why 10

Parag Mundhada - 7 years, 10 months ago

Log in to reply

0 is a multiple of 3. i.e. 3*0=0

puzzle seeker - 7 years, 10 months ago
Sherry Sarkar
Jul 21, 2013

The digits divisible by 3 are 0, 3, 6, and 9. So every 10 numbers (1 - 10 or 41 - 50) we have 6 allowed numbers.

We also have to consider which groups of ten we can count. For example, 61 - 70 would not work because it has the tens digit 6 which is not allowed (70 does not have the digit 6, however it does have the digit 0 so we can take the whole group of 10 out).

Dividing 100 by 6, we get 16 with a remainder of 4. Counting out 16 approved groups of 10 we arrive to the 17th approved group to count the last four numbers on the list. The 100th number is 255

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...