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Subtract a 1-digit number from a 2-digit number with regrouping

This understanding involves decomposing a two-digit minuend by exchanging one ten for ten ones when its ones digit is less than the one-digit subtrahend, then finding the difference while preserving the quantity. Place-value drawings, equations, and number lines can show that the regrouped number is equivalent to the original and help prevent treating the tens and ones digits as independent; this reasoning supports subtraction with larger numbers.

Detailed Explanation: Subtract a 1-digit number from a 2-digit number with regrouping

When the ones digit is smaller than the number you are subtracting, regroup one ten as ten ones.

Example: subtract 77 from 4242.

  1. Write the numbers by place value:

    • 4242 has 44 tens and 22 ones.
    • We cannot subtract 77 ones from 22 ones.

  2. Exchange 11 ten for 1010 ones:

    • 44 tens becomes 33 tens.
    • 22 ones becomes 1212 ones.

    The amount is still the same because

42=3 tens+12 ones=30+12.42=3\text{ tens}+12\text{ ones}=30+12.

  1. Subtract the ones:

12−7=512-7=5

  1. Subtract the remaining tens:

3 tens=303\text{ tens}=30

  1. Put the tens and ones together:

30+5=3530+5=35

So,

42−7=35.42-7=35.

Learn by doing: Subtract a 1-digit number from a 2-digit number with regrouping

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    Subtraction - Whole Number 2 x 1, With Borrow

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