Mentally Add 100 to a Number from 100 to 900

Students mentally add 100 to a three-digit number by increasing the hundreds digit by one while keeping tens and ones unchanged (e.g., 100 more than 347 is 447).

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

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12 rungs. A star at the top.

36 + 100 =

How it is taught

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One hundred more raises the hundreds digit.

Tens and ones stay the same.

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.

349 + 100 = ___ — 449. 1 jump of 100 from 349 lands on 449 — 1 mark along, not 100 little steps.

347 + 100

Let's find 100 more than 347. There's a quick mental trick — watch closely.

First, line up the digits by place value: hundreds, tens, and ones.

Now we add 100. Notice the 1 sits in the hundreds place, with zero tens and zero ones.

Here's the secret: only the hundreds digit changes, because we're adding one whole hundred.

Three hundreds plus one hundred makes four hundreds. Write a four.

The tens digit stays the same — still four.

The ones digit stays the same too — still seven.

Grade 2 · 100 more than a 3-digit number

So 100 more than 347 is 447. To add 100, just bump the hundreds digit up by one!

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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