Positives and Negatives

Now that I have been teaching mental arithmetic for a while I
have noticed differences between the acquistion of adding
and subtracting strategies. As you would predict, the addition
strategies were easier for students to learn and have success
with this method of calculation. The subtraction strategies
were difficult for some students to use. They became confused
about some of the strategies that we were learning, especially
the 'count on' and 'count back'. Perhaps this was due to the fact
that some students still did not have a complete understanding
of the arrangement of numbers on a number line and that
subtraction is 'going backwards'.

Another critical difference that I noticed, especially for the
subtracting strategies was that if the student did not have
their basic facts memorized they did not have success with
mental arithmetic. The 'counting on their fingers' and using
a hundreds chart to assist with mental arithmetic seemed to
add to their confusion. Consequently, I have withdraw
teaching these strategies.

One positive observation to consider is that students with
their facts memorized became very quick at using mental
arithmetic and I had to keep sharp to stay with them.
That was very pleasing to see. Sometimes I learn as
much as they do.

Moving forward now will require teaching the strategy of
'borrowing and carrying' so students will have success with
adding and subtracting two and three digit numbers. This is
not a mental arithmetic strategy, but it is a valid and workable
strategy for calculation.

Liza