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'Two and Two' printed from http://nrich.maths.org/
Why do this problem?
offers an opportunity to practise addition in a more interesting and challenging context than is usual. It requires students to work systematically, record their progress efficiently and apply their understanding of place value.
Write the following sum on the board and ask students to complete it.
$\; \; \; 8 \; 6 \; 1$
$+ \; ? \; ? \; ?$
$1 \; 4 \; 2 \; 7$
Expect justifications for any suggestions.
This should be unproblematic so move onto a more challenging problem.
If each letter stands for a distinct digit what are the values of $a$, $b$ and $c$?
$\; \; \; b \; b$
$+ \; c \; b$
$a \; c \; c$
"How can we approach this?"
Expect fairly random suggestions to start with but aim to use the discussion as an opportunity to model working systematically.
"What happens if $b$ is $1, 2, 3, \ldots$ ?" - rejecting values as soon as it is apparent they do not work and discussing how you know.
"How can we be sure we have all the solutions?"
Set the students off in pairs to work on the problem TWO + TWO = FOUR .
Establish that you are not going to announce how many solutions there are and that you will expect students to work systematically and be able to justify that they have all the possible solutions.
Are we certain we have considered all the possibilities?
Which other word sums work? - eg
ONE + ONE = TWO
FOUR + FIVE = NINE
Which definitely don't work? - eg
ONE + TWO = THREE
ONE + THREE = FOUR
Suggest students start with Spell by Numbers