# Package 5-3 It’s time to get magical

Magic tricks provide great opportunities for learners to develop mathematical reasoning and practice their skills in mental computation. This trick is helpful for practising addition and subtraction facts.

# Things you need

Ideal Back up

Pencils or markers

Paper

Numbered cards

# Why is this activity important?

Magic tricks provide great opportunities for students to develop mathematical reasoning and practice their skills in mental computation. This trick is helpful for practising addition and subtraction facts. It also helps students develop skills in choosing efficient strategies for solving addition and subtraction problems. Practicing the magic trick multiple times helps students develop confidence, consolidate skills in mental computation and helps them see the maths underlying the magic.

# Before you start

You need some pens and a piece of paper.

Dice are required for the optional activities.

# What your child needs to know and do

Your child needs to be able to add 1 and 2-digit numbers.

# What to do next

• Choose any number on the grid.

• Write it down.

• Write down a second number… BUT… it has to be a different row and different column to your first number

• Record a third number… it has to be a different row and different column to your first two numbers

• Write down a fourth number… it has to be a different row and different column to your first three numbers.

• We found out that

34 = 1 + 6 + 11 + 16; and

34 = 13 + 12 + 6 + 3

• Now we wonder... is the sum always 34?

Activity too hard? Activity too easy?

Try using a different strategy to do the calculations for the magic trick. For example, you might like to use a calculator. Then, work backwards to think about what mental strategies you could have used to solve the problem.

Complete several times to practice using mental strategies with 2-digit numbers.

Investigate what happens with this magic trick if you created a 5 x 5 table.

Work together to explore which mental computation strategies are the most efficient when adding and subtracting.

You can also work together to investigate how this magic trick works!

If you had a grid with different numbers in it, does this trick work the same? What number do those numbers add up to? Is it 34?

Try adding 4 numerals together. Eg. 5 + 6 + 11 + 17 = ?

How can we add these numbers efficiently? What strategies did you use?

One strategy you might try is to add the tens first and then add the ones.

5 + 6 + 11 + 17 = 10 + 10 + 5 + 6 + 1 + 7

= 20 + 5 + 6 + 1 + 7

= 20 + 10 + 1 + 1 + 7

= 30 + 1 + 1 + 7

= 39