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One and Two digit Addition and
Subtraction
Peter Gibby
Counting
• We are all familiar with the number line, we
start at 1 then go to 2 then 3, 4, 5 and so on.
• Addition is just counting up so many times.
• We use the plus sign (+) to represent addition.
• So if we add one and one, 1+1, we get 2.
• We use the equals sign (=) to represent the
new value.
• Every number has a number before and after
it so any two numbers can be added together,
even larger numbers.
• 5+3=8
• To get that we counted 3 values higher than 5
– 5, 6 (1), 7 (2), 8 (3)
• Notice that we don’t count the number 5, just
the 3 numbers following it.
Subtraction
• Subtraction is just backwards addition or
counting.
• We start at a larger number and take a value
out of it. We use the subtraction sign (-) to
represent this.
• We know that 3+2=5, so 5-2=3 because
– 5, 4 (1), 3 (2)
• Notice that we still don’t count the number
we start on
Two Digit Numbers
• Counting to 9 is great, but there are a lot more
numbers than that. We use our ten digits to
represent all these numbers.
• When we see a two digit number we know
that the first value is the tens place. This
means instead of representing a value of one,
it takes a value of ten.
• What we do is we count by tens when we use
the first digit, so
– 14 is the same thing as ten plus four (10+4=14)
– 26 is the same as, 10 (1), 20 (2), twenty plus 6
(20+6=26)
Adding Two Digit Numbers Together
• When we add two digit numbers together, we
count based on whether it was in the ten’s
place or the one’s place, so
– 23+14=
• 2+1=3
• 3+4=7
• 37 (always put the ten’s place first)
Subtracting Two Digit Numbers
• Subtracting is still just the opposite of adding.
Instead of going up, we go down. We know
that 12+15=27 so,
– 27-15=12
– If you can’t see how we got that you can just
count backwards for each digit,
– 27-15
• 2-1=1
• 7-5=2
• 12 (We still keep the ten’s place first)