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

Students will mentally add 10 or 100 to any given number between 100 and 900. They will recognize that adding 10 changes only the tens digit and adding 100 changes only the hundreds digit, while other digits stay the same.

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

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

0 of 12

12 rungs. A star at the top.

36 + 10 =

How it is taught

0:00 of 3:23

Add 10 to the tens place.

Add one hundred to the hundreds place.

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.

  1. Find where you start on the line.
  2. Hop 10 or 100 at a time: forward to add, back to take away.
  3. 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.

249 + 100 = ___ — 349. 1 jump of 100 from 249 lands on 349 — 1 mark along, not 100 little steps.

342 + 10 and 342 + 100

When we add ten or one hundred, only one digit changes. Let's see the trick.

What changes when we add 10 or 100?342hundredstensones+10=352342+100=442Only ONE digit changes!RULE+10 or+100one digit changes
Grade 2 · Adding 10 changes the tens; adding 100 changes the hundreds

Here's our number: three hundred forty-two. Three hundreds, four tens, and two ones.

First, let's add ten. Three hundred forty-two plus ten.

Adding ten only changes the tens digit. Four tens become five tens.

Three hundred forty-two plus ten is three hundred fifty-two. The hundreds and ones don't change.

Now let's add one hundred to the same number.

Adding one hundred only changes the hundreds digit. Three hundreds become four hundreds.

Three hundred forty-two plus one hundred is four hundred forty-two. The tens and ones stay the same.

Add ten, change the tens. Add one hundred, change the hundreds. Just one digit changes!

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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