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IX.Estimate Products
Toestimate a product by rounding:
- Round each factor to its greatest place.
- Multiply the rounded factors.
Estimate: 365 4840
400 5000 200,000
Both factors were rounded up, so 365 4840 is less
than 200,000.
Estimate:10.25 0.87
10 0.9 9
One factor is rounded down and the other is rounded up.
So 10.25 0.87 is close to 9.
X.Estimate Quotients
You can estimate quotients using compatible numbers,
numbers that are easy to compute with. Two whole numbers
are compatible if one divides the other evenly.
Estimate: 1895 48
about about
2000 50
So 1895 48 2000 50 40.
XI.Multiply Whole Numbers.
To multiply by a two-digit number, multiply by ones, then by tens.
Add the partial products.
To multiply by a three-digit number, multiply by ones, then by tens,
then by hundreds. Add the partial products.
Think
6 5, so 3 hundreds rounds up
to 4 hundreds.
8 5, so 4 thousands rounds up
to 5 thousands.
Think
0 5, so 1 ten stays as 1 ten
and 10.25 rounds down to 10.
7 5, so 8 tenths rounds up
to 9 tenths.
Estimate: 23.50 5.65
about about
24 6
So 23.50 5.65 24 6 4.
Multiply: 126 78
Estimate Compute
Remember:
The symbol means
“is approximately equal to.”
24
17
168
24
408
78
126
80
100
8000
24
17
20
20
400
Multiply: 17 24
Estimate Compute
78
126
468
156
78
9828
