Adding and Subtracting
decimals!
Mr. Vachon says, “If you can carry,
borrow, and line up place values… I
bet you can add and subtract decimals
as fast as a jack rabbit!”
Remember that for our lessons…
Is 1 whole
Like a dollar!
Is 0.1
Or 1/10
Like a dime!
Is 0.01
Or 1/100
Like a penny!
Remember when we add decimals, it’s not very
different than adding regular numbers.
+
0.45
+
=
0.26
Mr. Vachon says, “First combine your hundredths!”
=
Let’s get to it!
1
0.4 5
+0.2 6
0. 71
+
1st – Put together your hundredths
2nd – If there are more than 10 of them, regroup and adjust
3rd – Put together your tenths
4th – If you have 10/10 then you’ve made one whole…Do We?
Let’s visually see what happens when we need to
borrow while subtracting decimals…
0.61
-0.25
0.61
__
0.25
=
“How can we possibly take .05 away from .01 ?” asked the confused student.
“We’ll just have to borrow like we always do,” said the teacher.
Looks like it’s time to borrow…Here’s what
we’re actually doing…
0.6 (1)
-0.2 5
“First we need to subtract our hundredths…but there aren’t enough!” whined the students.
“Looks like we’ll have to borrow from one of those big tenths,” said Mr. Vachon smugly.
Break one of those tenths (rods) into hundredths (cubes) and the problem will
be very easy.
Let’s Get To It!
0.5 (11)
-0.2 5
0. 3 6
*Now that we’ve already borrowed, and regrouped from the tenths,
what are we left with?*
1st – Start one the far right, with the hundredths.
2nd – Subtract 0.05 (five hundredths) from 0.01 (one Hundredth)
3rd – Now subtract 0.2 (2 tenths) from 0.5…What’s left?
4th – Any whole ones to deal with?