17. 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?
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35 is the correct answer
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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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