Question

20. There are 105 numbers written in a row: 1,2,2,3,3,3,4,4,4,4,5,5,5,5,5.... How many of these numbers are divisible by 3?​

Answer 1

Given  : There are 105 numbers written in a row: 1,2,2,3,3,3, 4, 4, 4, 4, 5, 5, 5, 5,5 ..

To Find :   How many of these numbers are divisible by 3

Solution:

1 is 1 tim2 , 2 is  2 times ,    n is n times

1 + 2 + 3  + ______ + n  =  105

n is the last digit

n(n + 1)/2  = 105

n(n + 1)  =  210

n = 14

Hence numbers are upto 14

numbers divisible by 3

3 , 6 , 9 , 12

3 + 6 + 9 + 12  = 30

30 numbers are divisible by 3

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