Decimal Operations

Decimals are just fractions with denominators $10, 100, 1000$. The columns after the point are tenths, hundredths, thousandths.

Adding and subtracting: line up the decimal points so that tenths sit under tenths.

(Writing $2.7$ as $2.70$ and $8.6$ as $8.60$ keeps the columns tidy.)

Multiplying by a whole number works like ordinary multiplication, keeping the point in line: $12.50 \times 4 = 50.00$. Multiplying by $10$ shifts every digit one place left: $3.6 \times 10 = 36$.