Divisibility

problem

Eminit

Age

11 to 14

Challenge level

The number 8888...88M9999...99 is divisible by 7 and it starts with the digit 8 repeated 50 times and ends with the digit 9 repeated 50 times. What is the value of the digit M?
problem

Just Repeat

Age

11 to 14

Challenge level

Think of any three-digit number. Repeat the digits. The 6-digit number that you end up with is divisible by 91. Is this a coincidence?
problem

Three Times Seven

Age

11 to 14

Challenge level

A three digit number abc is always divisible by 7 when 2a+3b+c is divisible by 7. Why?
problem

Book Codes

Age

7 to 11

Challenge level

Look on the back of any modern book and you will find an ISBN code. Take this code and calculate this sum in the way shown. Can you see what the answers always have in common?
problem

Gran, How Old Are You?

Age

7 to 11

Challenge level

When Charlie asked his grandmother how old she is, he didn't get a straightforward reply! Can you work out how old she is?
problem

Adding All Nine

Age

11 to 14

Challenge level

Make a set of numbers that use all the digits from 1 to 9, once and once only. Add them up. The result is divisible by 9. Add each of the digits in the new number. What is their sum? Now try some other possibilities for yourself!
problem

Flow Chart

Age

11 to 14

Challenge level

The flow chart requires two numbers, M and N. Select several values for M and try to establish what the flow chart does.
problem

Oh! Hidden Inside?

Age

11 to 14

Challenge level

Find the number which has 8 divisors, such that the product of the divisors is 331776.
problem

AB Search

Age

11 to 14

Challenge level

The five digit number A679B, in base ten, is divisible by 72. What are the values of A and B?
problem

Square Routes

Age

11 to 14

Challenge level

How many four digit square numbers are composed of even numerals? What four digit square numbers can be reversed and become the square of another number?