Module 5 · Place value and decimals · Lesson 22 of 39
Decimal arithmetic
Line up the points to add and subtract; count places to multiply; shift to divide.
In this lesson · about 16 minutes
- Add and subtract decimals
- Multiply decimals
- Divide by a decimal
Add and subtract: stack the numbers with the decimal points directly on top of each other, bring the point straight down, and work column by column like whole numbers. 13.16 + 8.74 = 21.90 and 13.16 − 8.74 = 4.42.
Multiply: ignore the points and multiply. Then count the digits after the point in both numbers, and put that many digits after the point in the answer. 13.1 × 8.74: 131 × 874 = 114,494, and 1 + 2 = 3 places, so the answer is 114.494.
Divide by a decimal: move the point in the divisor until it's a whole number, and move the point in the dividend the same number of places. 13.1 ÷ 8.74 becomes 1310 ÷ 874 ≈ 1.4989. When it doesn't come out even, stop after a few places and round.
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