Addition and subtraction

problem
Reverse subtraction

Age

11 to 14

Challenge level

What is the value of c in this subtraction?
problem
Alphabetti Sudoku

Age

11 to 16

Challenge level

This Sudoku requires you to do some working backwards before working forwards.
problem
Mobile numbers

Age

5 to 11

Challenge level

In this investigation, you are challenged to make mobile phone numbers which are easy to remember. What happens if you make a sequence adding 2 each time?
problem
Different magic square

Age

11 to 14

Challenge level

Can you solve this magic square?
problem
Debt recovery

Age

11 to 14

Challenge level

Tony and Tina can't work out which of them owes what to the other. Can you?
problem
Little Goldbach

Age

11 to 14

Challenge level

How many of the numbers 1 to 20 are not the sum of two primes?
problem
Mathematical ages

Age

11 to 14

Challenge level

Arrange these three famous mathematicians in order with the shortest-lived first.
problem
Bus route

Age

11 to 14

Challenge level

How far apart are the first and last bus stops in this bus route?
problem
Highs and lows

Age

11 to 14

Challenge level

What were the maximum and minimum temperatures recorded in my max/min thermometer?
$Build it \Up more$

problem
Build it up more

Age

7 to 11

Challenge level

This task follows on from Build it Up and takes the ideas into three dimensions!