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Double Digit

Choose two digits and arrange them to make two double-digit numbers. Now add your double-digit numbers. Now add your single digit numbers. Divide your double-digit answer by your single-digit answer. Try lots of examples. What happens? Can you explain it?

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Choose any 3 digits and make a 6 digit number by repeating the 3 digits in the same order (e.g. 594594). Explain why whatever digits you choose the number will always be divisible by 7, 11 and 13.

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Digit Sum

What is the sum of all the digits in all the integers from one to one million?

Cycle It

Age 11 to 14 Challenge Level:

The following solution was sent in by Julia of Downe House School. Well done Julia!

The total reached when the complete cycle of numbers are added together must always be the same because:

At one point in the cycle, each number (1 to 9) will be in each and every digit position ( e.g., units, tens, hundreds, thousands etc. ). In the first position for example, whatever number is there in the first place, by the end of the cycle all the numbers from 1 to 9 will have been there and so the total in that column will always be the same (the sum of the numbers 1 to 9, i.e. 45). The same is true for each column or digit position so the overall total will always be the same. The units digit is 5, then 4 is carried into the tens place to make 49, so the tens digit is 9 and 4 is carried into the hundreds place ... and so on.

The total is 4 999 999 995.