Mentally Subtract 10 or 100 from a Number from 100 to 900

Students will mentally subtract 10 or 100 from any given number between 100 and 900. They will identify that subtracting 10 reduces the tens digit by 1 and subtracting 100 reduces the hundreds digit by 1.

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Grade 2 · Mental Math: Adding and Subtracting 10 and 100 · question 1 of 3

0 of 12

12 rungs. A star at the top.

76 − 10 =

How it is taught

0:00 of 3:21

Minus 10 hits tens.

Minus 100 hits hundreds.

Read it: Hop by ten or a hundred

You are on 30 on the number line. 3 hops of ten forward land on 60. A hop of a hundred works the same way: it lands on the next hundred.

Find where you start on the line.
Hop 10 or 100 at a time: forward to add, back to take away.
Count the hops: 30 + 30 = 60.

Counting 31, 32, 33 all the way to 60 works, but it is slow and easy to lose track.

Each hop lands on the next tens or hundreds tick, so you skip the numbers in between.

You already know 30 plus 10 is 40, so a hop replaces counting 31 through 39.

To add or take away tens or hundreds, hop 10 or 100 at a time along the line.

Making three hops but landing on 50, because you counted 30 as a hop.

30 is where you start. The first hop lands on 40, then 50, then 60.

343 − 100 = ___ — 243. 1 jump back of 100 from 343 lands on 243 — 1 mark along, not 100 little steps.

540 − 10 and 540 − 100

Mental math trick! When we subtract ten or one hundred, only one digit changes.

Here's our number — five hundred forty. Five hundreds, four tens, and zero ones.

First, let's subtract ten.

Subtracting ten only changes the tens digit. The hundreds and ones stay the same!

Four tens minus one ten is three tens, so the tens digit becomes three.

The hundreds and ones don't change. Five hundred forty minus ten is five hundred thirty!

Now let's subtract one hundred from five hundred forty.

Subtracting one hundred only changes the hundreds digit. The tens and ones stay the same!

Five hundreds minus one hundred is four hundreds, so the hundreds digit becomes four.

The tens and ones don't change. Five hundred forty minus one hundred is four hundred forty!

Grade 2 · Mental math with place value

Subtract ten? Tens digit drops by one. Subtract one hundred? Hundreds digit drops by one. That's all you need!

What you see

While they climb, your page writes one true line to ask about tonight, and a short note each Sunday.

MayaGrade 3 · Mathematics

Did you know?

A chessboard has 64 squares — share them out between the 8 rows and each row holds exactly 8.

Today’s goal: 1 star — one skill mastered, every rung of its ladder climbed
Climbing now: Divide within 100

An example student.

MayaGrade 3 · Mathematics

Maya earned 2 stars this week — Interpret multiplication as equal groups, which took 6 answers over 3 days before its star landed 3 days ago and Multiply by 6, 7, 8 and 9. Next up: Divide within 100.

An example week.

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