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Subtracting Across Zeros

The Hook

The trickiest move.

What is it?

Borrowing when the neighbor is a Zero (empty).

Time Travel (History)

A specific challenge that highlights the importance of Place Value systems.

Cheat Code

Make the 0 a 9 and keep moving left.

Practice This Now!

The Field Guide

In Plain English: If you need sugar but your neighbor has none, they have to ask THEIR neighbor. You have to keep moving left until you find a non-zero number, then pass it down the line.

In The Real World: Paying a $1.50 toll with a $100 bill.

How To Do It

  1. Go left until you find a number > 0.
  2. Borrow 1 from it.
  3. Turn every '0' you passed into a '9'.
  4. Add 10 to your starting number.
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Booby Trap!

Skipping the middle zero. You can't jump over neighbors!

Real World Challenge
"A car odometer reads 1000 miles. You roll it back 7 miles. What does it say?"
The Logic: 1000 - 7. Borrow from the 1. The 0s become 9s. Last 0 becomes 10. 10-7=3. Answer 993.

Training Drills

Chain Reaction
4 9 10 5 0 0 - 3 2 4 ------- 1 7 6
500 becomes 400 + 90 + 10.