Problem
Skip-count backward to the start
Starting from an unknown number ■ and skip-counting up by 10 four times lands on 6792. Find the starting number ■.
Base-ten numbers
Your answer
Watch out The rule counts up by 10, but to find the start you go backward — subtract, don't add.
How to solve
Strategy Work Backwards — The end result is given and we want the start, which is the classic signal to work backwards: undo each step of 10 by counting back from 6792.
1STEP 1
Find the total amount added
Four jumps of 10 add 10 four times — that's 40 in all.
4 × 10 = 40
Four jumps of 10 stack up to 40, just like skip-counting 10, 20, 30, 40.
2.NBT.A.2Look For A Pattern2STEP 2
Count back from the ending number
Undo the +40 by counting back from 6792: 6792, 6782, 6772, 6762, 6752.
6792 - 40 = 6752
To undo adding 40, you subtract 40, landing on where you began.
4.NBT.A.2Work BackwardsYou find the starting number by subtracting 40 from 6792, because 40 is exactly what the skip-counting added on.
Why?
The four counts up by 10 piled a total of 40 onto the start, so 6792 is just the start with 40 added; taking that same 40 back off returns to the start.
Why?
Removing exactly the amount that was added lands you back where you began, since taking away reverses adding on.
🧱Inverse operationsSubtraction undoes addition and division undoes multiplication — they reverse each other.
Why?
The amount to take back off is 40 because four jumps of 10 add up to 40.
Why?
Four jumps of 10 are four equal groups of 10, and adding 10 four times makes 40.
🧱Multiplication as equal groupsa times b means a equal groups of b — it is just repeated adding of the same amount.
Answer
6752
6792 − 40 = 6752
Check forward: 6752 + 10 = 6762, +10 = 6772, +10 = 6782, +10 = 6792. Four jumps of 10 from 6752 reach 6792, so the start is right.
Takeaway
When you know where you ended up, just hop backwards the same number of tens to find where you started!
- Find the total amount added
- Count back from the ending number
Where next?
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Step back and solidifysuggested