Let's talk in some details about subtracting numbers in real life. If you have 100 pennies, you have one dollar - $1.00. On the way home, you loses one penny. Now you have 99 pennies or 100 pennies minus 1 penny: 100 - 1 = 99. Now let's take the next step and show mathematically how to do what we just talked about.

Step 1
First, state the problem. We have 100 pennies and we loses 1 penny which leaves us 99 pennies. |
Column CBA 100 -1 ——— |

Step 2
We can not subtraction 1 from zero in column A. So we borrow 1 from column C. The borrow one is moved one place to column B. But it must be in column A to solve the problem. So we borrow one from column B. Let's look at step 3. |
Column CBA 11 100 -1 ——— |

Step 3
Now subtract the value 1 in column A from the carried 10 in column A. In column B, 9 minus nothing (actually zero) is 9. Carry carried value 9 in column B to the answer line. |
Column CBA 9 10 100 -1 ——— |

Step 4
Now that we have borrowed from column C, we can solve the problem.The final problem answer is 100 pennies minus one penny is 99 pennies. |
Column CBA 100 -1 ——— 99 |

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