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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?

Not a Polite Question

When asked how old she was, the teacher replied: My age in years is not prime but odd and when reversed and added to my age you have a perfect square...

Whole Numbers Only

Can you work out how many of each kind of pencil this student bought?

Arrange the Digits

Age 11 to 14
Challenge Level

The closest total is 1503.

The first two correct answers to this problem were received from Natalie and Jake, both from the West Flegg GM Middle School. Both had arrived at the answer by trial and improvement techniques.

Later on, work by Thomas from Wymondham High School arrived. He had been a little more systematic in his search and explained how he strove initially to get the hundreds column to total 15 and the tens column to total 9. Unsuccessful at this he moved on to consider making the units column at least 20 and the tens column eight, which after a little searching gave him a result that he was satisfied with.

In whatever way the digits 1 to 9 are arranged and added together the total will always have a digital root that is 9.

e.g. 123 + 45 + 678 + 9 = 855

where 8 + 5 + 5 = 18, i.e the sum of the digits

and 1 + 8 = 9

123 + 456 + 789 = 1368

and the sum of the digits is 1 + 3 + 6 + 8 = 18

and 1 + 8 = 9

When restricted to three 3-digit numbers, they too must have a total whose digital root is 9.

Hence, 1500 cannot be a solution because its digital root is 6. While the integers 1494 and 1503 both have digital roots of 9, 1503 is nearer to 1500 than 1494.

Search for the digits which would make the units add up to 13 or 23, the tens digits add up to 9 or 8 and the hundreds digits add up to 13 or 14 e.g. 519 + 748 +236 = 1503.

See the article Divisibility Tests.