# Lesson Video: Subtracting Tens on a Number Line Mathematics • 1st Grade

In this video, we will learn how to subtract multiples of ten from a two-digit number using a number line with numbers up to 100.

11:12

### 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.

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