perplexus.info :: Just Math : How Many 21's

How Many 21's (Posted on 2024-09-14)

List all integers from 1 to 999, one after another to form a number

12345678910111213...997998999

How many times does the "21" grouping appear in this number?

No Solution Yet Submitted by Danish Ahmed Khan

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solution and check

Comment 1 of 1

First 21 itself of course. Then x21 where x is 1 through 9. And there's 210 through 219.

There's the less obvious 12 13 combo and 1x2 1x3 where x is 0 through 9.

That's 1 + 9 + 10 + 1 + 10 = 31.

s='';

for i=1:999

s=[s num2str(i)];

end

f=strfind(s,'21');

length(f)

verifies this answer.

Posted by Charlie on 2024-09-14 10:52:13

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