BOX 7.2
How Many Altogether?
The teacher begins with a request for an example of a basic computation.
Teacher: Can
anyone give me a story that could go with this multiplication . . .12 x
4?
Jessica: There
were 12 jars, and each had 4 butterflies in it.
Teacher: And if I
did this multiplication and found the answer, what would I know about
those jars and butterflies?
Jessica: You'd
know you had that many butterflies altogether.
The teacher and students next illustrate Jessica's story and construct a
procedure for counting the butterflies.
Teacher: Okay,
here are the jars. The stars in them will stand for butterflies. Now,
it will be easier for us to count how many butterflies there are
altogether, if we think of the jars in groups. And as usual, the
mathematician's favorite number for thinking about groups is? [Draw a
loop around 10 jars.]
Sally: 10.
The lesson progresses as the teacher and students construct a pictorial
representation of grouping 10 sets of four butterflies and having 2 jars
not in the group; they recognize that 12 x 4 can be thought of as 10 x 4
plus 2 x 4. Lampert then has the children explore other ways of
grouping the jars, for example, into two groups of 6 jars.
The students are
obviously surprised that 6 x 4 plus 6 x 4 produces the same number as 10
x 4 plus 2 x 4. For Lampert, this is important information about the
students' understanding (formative assessment--see Chapter 6). It is a sign that she needs to do many
more activities involving different groupings. In subsequent lessons,
students are challenged with problems in which the two-digit number in
the multiplication is much bigger and, ultimately, in which both numbers
are quite large--28 x 65. Students continue to develop their
understanding of the principles that govern multiplication and to invent
computational procedures based on those principles. Students defend the
reasonableness of their procedures by using drawings and stories.
Eventually, students explore more traditional as well as alternative
algorithms for two-digit multiplication, using only written symbols.