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This problem is designed to be done in a group. So, ideally, you need to find three or four other people to work with you.

Firstly, take some time to work on your own to find the answers to these calculations, without writing anything down. (Although it would be good to write down the answer itself so you don't forget it.)

$19 \times 24$
$198 + 997$
Half of $57.6$
$3841 - 665.3$
$5.2 \div 4$
$101 \times 16 \times 4$

Now, join together with the other people in your group and focus on the first question.

Do you all agree on the answer?
Give EVERYONE in the group time to explain how they worked it out.
As a group, decide whose method you think is most efficient and why.

Do the same for each of the six questions: give everyone the chance to explain their own method and then choose the most efficient for that calculation.

For the final bit of the challenge, you will need a set of these cards - one set for the group. Each card has another calculation on it.
As a group, decide on the most efficient method for each of the cards.
What did you decide and why?