# Mental calculations

**95**NRICH Mathematical resources connected to

**Mental calculations**

problem

### Mathdoku

Complete the Mathdoku grid using the clues. Can you convince us that the number you have chosen for each square has to be correct?

problem

### An Easy Way to Multiply by 10?

Do you agree with Badger's statements? Is Badger's reasoning 'watertight'? Why or why not?

problem

### Arranging Additions and Sorting Subtractions

Order these four calculations from easiest to hardest. How did you decide?

problem

### Subtraction Slip

Can you spot the mistake in this video? How would you work out the answer to this calculation?

problem

### Compare the Calculations

Can you put these four calculations into order of difficulty? How did you decide?