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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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