If we go on writing even positive integers one after the other in ascending order we will get a number that looks like -
2468101214161820222426283032343638404244 … … … …
We need to identify 120th digit of this number.
If we observe, numbers from 2 to 8 (2, 4, 6 and 8) they are single digit even numbers. While numbers from 10 to 98 are two-digit even numbers. Means, each two-digit even number will account for two digits in the given number and further digits will be given by 3-digit even numbers and so on.
Hence, in the given number
Digits occupied by 1-digit even numbers =4
2468101214161820222426283032343638404244 … … … …
Out of 120 digits to be counted, 4 are by single digit numbers. Remaining 116 digits will be counted using 2-digit even and / or 3-digit numbers.
As each 2-digit even number accounts for 2 digits and we can see that from 10 to 98 there are 45 two-digit even numbers.
2468
1012
1416
1820222426283032343638404244 … … … …
10, 12, 14, 16, 18 … …. This is an Arithmetic progression
Here, First Term (a) = 10, Last term = 98 Common difference (d) = 2
Number of terms = [ (Last Term - First Term)/2) + 1] = [ (98-10)/2 +1] = 44 +1 = 45
These 45 2-digit numbers account for 90 digits.
Out of 116 remaining digits 90 are occupied by two-digit even numbers. Hence, Digits remaining to be counted = 116 - 90 = 26
These 26 digits will be counted now by counting digits of 3-digit even numbers.
2468101214161820222426283032343638404244 … …98
100102
104106 …
As each 3-digit even number will occupy 3 digits and we need to count 26th digit starting from 100
We divide 26 by 3.
We get, 26 = 3 x 8 +2
This means first 8 3-digit even numbers will account for 24 digits and middle digit of 9th 3-digit number (second digit) will be the 120th digit (answer required) of the number. Three-digit numbers in order are
100, 102, 104, 106, 108, 110, 112, 114,
116We can see that 9th 3-digit even number is 116 and its second digit = 1.
Hence, 120th digit of the given number = 1