Video Transcript
Subtracting 10s on a Number
Line
In this video, we will learn how to
subtract multiples of 10 from two-digit numbers using a number line with numbers up
to 100.
This is a number line. Our number line starts at zero. And on this number line, each
division is worth one. Let’s count forward in ones. Zero, one, two, three, four, five,
six, seven, eight, nine, 10. When we count forward in ones, the
numbers increase by one each time. Zero, one, two, three, four, five,
six, seven, eight, nine, 10. We can also count forward in 10s on
a number line. 10, 20, 30, 40, 50, 60, 70, 80, 90,
100. The numbers are increasing by 10
each time.
In this video, we’re learning how
to subtract multiples of 10 using a number line. When we’re subtracting using a
number line, we have to count back. Let’s practice counting back in
10s. We’re going to start at 100 and
count back in 10s. 90, 80, 70, 60, 50, 40, 30, 20, 10,
zero. If we wanted to subtract 30 from 70
on a number line, we would need to start at number 70 and count back 30 in 10s. We know the number 30 has three
10s. So we’re going to start at 70 and
count back three 10s. 10, 20, 30. We started at 70 and counted back
three 10s. 60, 50, 40. 70 subtract 30 is 40.
How would we calculate 68 subtract
50 using this number line? We’d need to start at number 68 and
count back 50. We know that 50 has five 10s. So we need to subtract five 10s
from 68. The number 68 is a two-digit
number. The six digit is worth six 10s. Six 10s take away one 10 leaves us
with five 10s. 68 take away 10 is 58. 68, 58, 48, 38, 28, 18. Did you notice how the tens digit
decreased by one each time? That’s because we’re counting back
in 10s. We’re taking away one 10 each
time. 68, 58, 48, 38, 28, 18. We’ve learned that when we subtract
a multiple of 10, such as 50, from a two-digit number, like the number 68, we can
count back in 10s on a number line.
Let’s use what we’ve learned to try
some practice questions.
You can use a number line to
subtract. Take away ones by counting back in
ones. 56, 55, 54, 53, 52. 56 subtract four equals 52. Take away 10s by counting back in
10s. 82, 72, 62, 52. 82 subtract 30 equals 52. Use number lines to find these
differences. 43 subtract six equals what. 54 subtract 40 equals what.
We’re shown two calculations and
two number lines. We’re asked to use the number lines
to help us find the difference. First, we have to find the
difference between 43 and six. In other words, we have to subtract
six from 43. The number 43 is already marked on
the number line. To subtract six, we need to count
back six. And we need to count back in
ones. 43, 42, 41, 40, 39, 38, 37. We started at 43, and we counted
back six on the number line. 43, 42, 41, 40, 39, 38, 37. 43 subtract six is 37.
Next, we have to use the number
line given to find 54 take away 40. The number 54 has two digits. The five digit is worth five
10s. We have to subtract 40. We know that 40 is four 10s, so we
have to subtract four 10s from 54. Let’s use the number line to
help. 54 subtract 10 is 44. 44 subtract 10 is 34. 34 take away 10 is 24. 24 take away 10 is 14. We started at 54 and counted back
four 10s. 44, 34, 24, 14 because 40 has four
10s.
We used number lines to find the
differences. To find 43 take away six, to find
43 take away six, we started at 43 and counted back six ones. 43 take away six is 37. And to find 54 subtract 40, we
started at 54 and counted back four 10s. 54 subtract 40 is 14.
Subtract 30 from 78. Use the number line to help
you.
In this question, we’re being asked
to subtract a multiple of 10, which is the number 30, from a two-digit number,
number 78. And we’re told to use a number line
to help. We can see that the number 78 has
already been marked on the number line. The number line also shows the
three 10s that we need to count back. This is because we’re subtracting
30. And 30 is three 10s. If we subtract 10 from 78, we’ll be
left with 68. Seven subtract one is six. Seven 10s take away one 10 leaves
us with six 10s. 68 subtract 10 is 58. 58 subtract 10 is 48. 78, 68, 58, 48.
We used the number line to subtract
30 from 78. The missing number is 48. 78 subtract 30 is 48.
Emma is subtracting on a number
line. Pick the calculation she is working
on. Is it 82 subtract 50, 82 subtract
55, or 82 subtract five? What is the answer?
We know that Emma is subtracting on
a number line. She starts at number 82. This is why the number at the start
of each calculation is the number 82. This is the number we’re starting
with. Emma is subtracting in 10s. She’s counting back by 10 each
time. She’s counting back 10, 20, 30, 40,
50. The calculation Emma is working on
is 82 take away 50. Next, we need to work out the
answer. What is 82 take away 50? We know that 82 is a two-digit
number. The eight digit is worth eight
10s. So if we take away a 10, the number
we’re left with is 72. 82, 72, 62, 52, 42, 32. We started at 82, and we counted
back five 10s. Five 10s are 50. 82 subtract 50 is 32. The answer is 32.
What have we learned in this
video? We’ve learned how to count back in
10s on a number line to subtract a multiple of 10 from a two-digit number.
