Write a sum for each number line model.
For review, if we are adding two numbers,
the two numbers we are adding
are called the addends,
the result of the addition is the sum.
Because both models have two arrows,
we know we have two addends,
where the first addend
is indicated by the
arrow starting at zero,
or the blue arrow.
And then the second addend is
indicated by the second arrow,
or the red arrow.
Looking at the first model,
because the blue arrow
starts at zero and jumps to two,
two is the first addend
which means we have two plus,
now we need to be careful
about the second addend.
Notice how the second arrow, or red arrow,
starts at two and jumps to nine.
Nine is not the second
addend, nine is the sum.
Which means we now know a
two plus the second addend
is equal to nine.
To determine the second addend,
indicated by the red arrow,
we need to determine
how many units we moved
from two to nine.
So starting at two, we count
the number of units to nine.
We have one, two, three, four,
five, six, seven.
We moved seven units to the right of two,
which means the seven
is the second addend.
Giving us the addition problem,
two plus seven equals nine.
Another way to think of this model
is two increased by seven is nine.
Looking at he second example,
you need to be a little bit
careful about the number line.
Notice how it's labeled every two units,
with tick marks every one unit.
For the first addend,
looking at the blue arrow
starting at zero, notice
how we start at zero
and jump to seven.
But is not labeled, but
let's go ahead and label it.
Seven is the first addend,
which means we have
seven plus the unknown
second addend equals,
looking at the red arrow, which
indicates the second addend,
notice how it ends at 18,
which is not the second addend,
it's the sum.
To determine the second
addend, we need to determine
how far we moved to the right
from the seven to get to 18.
So starting at seven,
we have one, two, three,
four, five, six, seven,
eight, nine, 10, 11 units,
which means 11 is the second addend,
giving us the addition problem,
seven plus 11 equals 18.
This model also shows seven
increased by 11 is 18.
I hope you found this helpful.
