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1 To 1,000,000,000

Riddle:

When the celebrated German mathematician Karl Friedrich Gauss (1777-1855) was nine he was asked to add all the integers from 1 through 100. He quickly added 1 to 100, 2 to 99, and so on for 50 pairs of numbers each adding to 101.

Answer: 50 X 101=5,050.

Now find the sum of all the digits in integers from 1 through 1,000,000,000.

That's all the digits in all the numbers, not all the numbers themselves.

Answer:

The numbers can be grouped by pairs:

999,999,999 and 0;

999,999,998 and 1'

999,999,997 and 2;

and so on....

There are half a billion pairs, and the sum of the digits in each pair is 81. The digits in the unpaired number, 1,000,000,000, add to 1. Then:

(500,000,000 X 81) + 1= 40,500,000,001.

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