This activity could just be a warm-up, to d
raw a class into some mathematical
thinking at the start of the lesson. Or it could be the first
example of a more open investigation based on place value.
If I did want to make this a more extended activity I might begin
with the "X marks the Spot
" puzzle as an
example, let the class together try it and discuss it, then
invite children to create a puzzle like this for themselves, but
to start with something simpler: Multiply two numbers, hide a
few digits, then swap with a friend. Try each other's puzzle,
check and discuss together, then share puzzles with other pairs.
There's lots to discuss, of course, but I would want to include
consideration of puzzles that have more than one solution.
If none arise naturally from the class, I'd ask them to find the
most digits that can be removed until a solution is no longer
Differentiation by challenge is easy enough to achieve. I can
control the size of the numbers that children use, and also vary
the balance between the use of reasoning and trial and error.
Notes on the construction of X marks the Spot :
For help with Spinners, look back at October 02
. Increment buttons are
very easy to insert and keep attention on the changing numbers,
instead of the keyboard.
I have used Conditional Formatting
to create a
colour change ( to bright red ) when the right answer occurs.
And have also used Conditional Formatting to conceal the
ten-millions digit if it's zero ( font colour matched to
background ) , and also to turn the border on or off for that
Cell K6 does the multiplication, before the digit boxes each
receive their correct value from the digits in that result.
Although column & row headers have been turned off in
Tools/Options/View, the location reference for the current cell can
still be seen beside the Formula bar, immediately above the
The individual digits are isolated using the INT function. INT
takes just the integer part of a value and ignores the rest, for
example INT of 3.845 is just 3. Using G2 as an example, this cell
calculates the hundreds digit with the formula : = INT ( $K$6 / 100
) - 10 * INT ( $K$6 / 1000 ) K6 was only made an absolute
reference, $K$6 , to help me copy a basic formula across all the
cells for digits and then just adjust the number of zeros required
in each formula. INT ( K6 / 100 ) calculates the number of whole
hundreds in the multiplication product. And from this I need to
subtract 10 times the number of whole thousands in the
This technique of isolating digits was also used in the