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Mentally Subtract 100 from a Number from 100 to 900

Students mentally subtract 100 from a three-digit number by decreasing the hundreds digit by one while keeping tens and ones unchanged (e.g., 100 less than 347 is 247).

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Mentally Subtract 100 from a Number from 100 to 900

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.

200 − 100 = .

How it is taught

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One hundred less lowers 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.

  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.

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

472 − 100

Let's find one hundred less than four hundred seventy-two — without paper!

Here is our number: four hundred seventy-two.

Each digit has a job: hundreds, tens, and ones.

When we subtract one hundred, only the hundreds place changes.

Four hundreds minus one hundred — how many hundreds are left?

Three hundreds! The four becomes a three.

The tens digit stays the same — still seven tens.

The ones digit stays the same too — still two ones.

100 less than 472 = ?Subtract 100 mentally472hundredstensones−1004 − 1 = ?372tens and ones stay the sameANSWER372100 less than 472
Grade 2 · One hundred less, in your head

So one hundred less than four hundred seventy-two is three hundred seventy-two. Easy in your head!

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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