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Two Many

Stage: 3 Challenge Level: Challenge Level:1

What is the least square number which commences with six two's? If you can't at first see how to do this with six twos try three twos. Clearly 222 is not a perfect square. There is no whole number between the square root of 2220 and the square root of 2229 but 149 lies between the square root of 22200 and the square root of 22299.

This was solved by Ling Xiang Ning, Allan, age 13 from Raffles Institution, Singapore and Tony Cardell and John Lesieutre, age 14 from State College Area High School, Pennsylvania, USA.

Here is Allan's solution: First I try 222222. It is not a square number.

Next, I try 2222220 and 2222229. I take the square root of both of them and find out that there is no square number between them.

Next, I try 22222200 and 22222299. I take the square root of both of them and find out that there is no square number between them...

...

...

I try 222222000000 and 222222999999. I take the square root of both of them and find out that there is one square number between them. This number is the square of 471405 which is 222222674025.

So the least square number starting with six twos is 222222674025