Learn with Amar
Teaching video
A teaching recording for this chapter has not been published yet. Start with the worked example below and explore the related animations where available.
Grade 2 chapters and video availability01 · Read and understand
What you will learn
- Undo a change when the final amount is known.
- Explain your method and check what the result means in the stated situation.
Before you start
Counting and comparing whole numbers, basic addition and subtraction, and recognizing simple shapes.
Keep paper nearby. Read the question once for the context, then again to identify what is known and what you need to find.
Start with a question
After 9 stickers are given away, 16 remain. How many were there at first?
Why this math matters
Reconstruct a starting balance or collection from its ending amount and known changes. The worked example shows how to connect the situation, its mathematical representation, and a check on the result.
Set up the model
A useful answer starts with clear assumptions:
- Use the units, grouping rules, and reference whole stated in the question.
- Treat the supplied counts and measurements as exact within the classroom model, unless an estimate or a random outcome is requested.
02 · Work through the example
Follow the reasoning, one step at a time.
Try to predict the next step before reading it. After each calculation, explain why the operation makes sense and how it helps answer the original question.
Work through it with Amar
See this example unfold.
The complete worked example, one idea at a time.
Work backward to find the start
PausedQuestion: Start with the question. Paused.
Question
Start with the question
After 9 stickers are given away, 16 remain. How many were there at first?
Before you calculate
Read what is known and what you need to find. Make a prediction before moving to the first calculation.
Starts paused. Play advances through the full text at a reading pace; pause whenever you need more time. Previous, Next, and the phase buttons let you set your own pace. Playback pauses when this walkthrough leaves the screen or you switch tabs.
Your device’s reduced-motion setting keeps each phase still. Manual controls remain available. The full written solution stays below.
Represent the situation
? − 9 = 16
The starting collection is the unknown.
Connect the parts
16 + 9
Put back the removed stickers to reverse the action.
State and check the result
25 stickers at first
Check: 25 − 9 = 16.
The result
25 stickers at first
Check: 25 − 9 = 16.
Common mistakes to catch
- The first number mentioned need not be the starting quantity.
- An undo operation reverses the story's change.
03 · Practice independently
Try it before revealing the answer.
Use paper or a calculator as needed. Write your units and reasoning, then open the hint or explanation to check your approach.
Practice 1
Some birds sit in a tree. Six arrive and now there are twenty. How many were there?
Show a hint
Undo the arrival.
Reveal answer and explanation
14
20 − 6 = 14, and 14 + 6 = 20.
Practice 2
A child writes 16 − 9 to solve the main story. What went wrong?
Show a hint
Sixteen is the end, not the start.
Reveal answer and explanation
They removed the stickers twice
To find the earlier amount, the removed nine must be restored.
Take the idea with you
Reconstruct a starting balance or collection from its ending amount and known changes.
04 · Reflect and continue
Can you explain it in your own words?
Before moving on, explain the main idea without looking at the worked example. Try both practice questions, check your reasoning, and name one mistake you now know how to avoid. Return to a step if you still need support.
Up next: Read an array by rows and columns
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