Using Estimation to Place the Decimal Point
Using estimation to place the decimal point is a Grade 5 math skill in enVision Mathematics, Chapter 3: Fluently Multiply Multi-Digit Whole Numbers. Students round each factor to a simple value, estimate the product, then use that estimate to determine where the decimal point belongs in the exact answer. This prevents common decimal placement errors in multiplication.
Key Concepts
To estimate the product of two numbers, round each factor to a nearby, easy to multiply number (like its greatest place value). The calculated product should be close to this estimate. If $A \approx A {est}$ and $B \approx B {est}$, then the exact product $A \times B$ should be reasonably close to the estimated product $A {est} \times B {est}$.
Common Questions
How do you use estimation to place a decimal point?
Round each factor to a nearby easy number and multiply to get an estimate. Your exact answer should be close to the estimate, so use it to decide where the decimal point goes.
Why might a student put the decimal point in the wrong place?
Students sometimes miscount decimal places. Estimating first gives a ballpark figure that makes it obvious if the decimal is off by a factor of 10 or more.
Give an example of using estimation to check a decimal product.
For 3.8 × 2.1, estimate 4 × 2 = 8. If your calculated answer is 7.98, the decimal is placed correctly since 7.98 ≈ 8.
Where is this skill taught in enVision Grade 5?
Chapter 3: Fluently Multiply Multi-Digit Whole Numbers in enVision Mathematics, Grade 5.
What is the relationship between estimation and exact calculation in decimal multiplication?
Estimation gives a reference point. The exact product should be reasonably close to the estimate, confirming the decimal point is in the right position.