Number and Operations in Base Ten: Fluently Adding and Subtracting Multi-Digit Whole Numbers (CCSS.4.NBT.4)

Regroup for ones: borrow $1$ ten so $18-9=9$ and tens become 5. Tens: $5-5=0$. Regroup for hundreds: borrow $1$ thousand so hundreds become $13$ (because $1$ thousand $=10$ hundreds), then $13-7=6$. Thousands: $3-2=1$. The difference is $1{,}609$. Mistakes often come from forgetting that $1$ thousand $=10$ hundreds or that borrowing $1$ ten gives $10$ ones.

Regroup across zeros: borrow $1$ thousand so hundreds become $10$ hundreds; borrow $1$ hundred so tens become $10$ tens; borrow $1$ ten so ones become $10$ ones. Compute: ones $10-6=4$, tens $9-5=4$, hundreds $9-2=7$, thousands $2-1=1$. The difference is $1{,}744$. Remember: borrowing $1$ thousand gives $10$ hundreds, and borrowing $1$ hundred gives $10$ tens.

Add ones: $8+9=17$, write 7 and regroup $10$ ones as $1$ ten. Tens: $6+5+1=12$, write 2 and regroup $10$ tens as $1$ hundred. Hundreds: $3+7+1=11$, write 1 and regroup $10$ hundreds as $1$ thousand. Thousands: $3+2+1=6$. The sum is $6{,}227$.

Ones: $3+7=10$, write 0 and regroup $1$ ten. Tens: $2+6+1=9$. Hundreds: $8+9=17$, write 7 and regroup $1$ thousand (since $10$ hundreds $=1$ thousand). Thousands: $5+1+1=7$. The sum is $7{,}790$. Errors like 7,780 or 6,790 come from missing a regroup: $10$ ones $=1$ ten and $10$ hundreds $=1$ thousand.