たし算とひき算

In column arithmetic, line up ones under ones and tens under tens so that matching places sit in the same column. If the columns are not lined up, the answer will be wrong, so this is the most important step. Always start calculating from the right-hand (ones) column.

When the ones digits add up to $10$ or more, you send one group of $10$ over to the tens column. This is carrying. For example, in $47+38$, the ones make $7+8=15$. Write the $5$ in the ones place and carry the $1$ (which means $10$) to the tens. Then the tens are $4+3+1=8$, so the answer is $85$.

When you cannot subtract in the ones column (the top digit is smaller than the bottom one), you borrow $10$ from the tens column. This is borrowing. For example, in $52-27$, you cannot do $2-7$. So borrow $1$ from the $5$ in the tens, making the ones $12$. Now $12-7=5$. The tens become $4-2=2$, so the answer is $25$.