Quiz

[Fellow Traveler]

Who want to be a millionaire had their Whiz Kids series and the asked a math question. So the kid had to figure this out entirely using his brain.
They gave the number 123456787700000 (actually they listed the last digits but I forgot what they were) and asked what was the square root. There were multiple choices a. 11111111, b. 1111111, c. 111111. How would you work this out in your head. The Whiz kid did. I have a procedure in mind but not the way he did it.

 

[marplots]

Who want to be a millionaire had their Whiz Kids series and the asked a math question. So the kid had to figure this out entirely using his brain.
They gave the number 123456787700000 (actually they listed the last digits but I forgot what they were) and asked what was the square root. There were multiple choices a. 11111111, b. 1111111, c. 111111. How would you work this out in your head. The Whiz kid did. I have a procedure in mind but not the way he did it.

Um... what was the question?

 

[Winterfrost]

Based on the example answers, let's assume the number was probably 123456787654321.

It's a (somewhat?) well-known math trick: take a number made up from a series of ones and square it. It will produce a palindrome 2n-1 digits long where n is the number of digits in the original number.

So 123456787654321 is 15 digits long. (15+1)/2 = 8. So the square root would be 111111111 (eight ones).

 

[BowlOfRed]

Given those choices, you don't have to worry about anything except the length. The given number starts with a 1, so it's pretty close to 10^14. The square root of that will be 10^7 (an eight digit number starting with 1). Pick the eight digit number.