Stuck? Start here. Read the steps, try the problem, then check your answer.
I can…
I can estimate the percent of a number using benchmark percents, rounding, and compatible numbers.
Key words
estimate — A reasoned answer that is close enough to be useful, found without exact computation.
benchmark percent — A friendly percent — 1%, 10%, 25%, 50%, 75%, 100% — used as a landmark for estimating.
compatible numbers — Numbers close to the originals that are easy to compute with mentally.
double number line — Two parallel number lines that pair amounts with their percents at the same positions.
the whole — The quantity that counts as 100% in a problem — and it can change between questions.
Decompose — Break a percent into friendly pieces that are easy to find.
Steps
The whole is 6 GB — that is 100%.
I mark the bottom line in benchmark jumps of 25%: 0%, 25%, 50%, 75%, 100%.
Equivalent ratios give the top line: 25% of 6 GB is 1.5 GB, so the top reads 0, 1.5, 3, 4.5, 6.
4 GB used lands between 4.5 (75%) and 3 (50%), close to 4.5 — so about 70% of the storage is used.
Try it
A family drove 298 miles on a trip, and 21% of the miles were on country roads. Estimate the miles on country roads using benchmarks or compatible numbers.
Check your answer
About 60
21% is about 20%, and 298 is about 300. 20% of 300 is 0.2 × 300 = 60.
Sentence starter
I know ___ because ___.
Goal: I can explain an estimate using the words about, benchmark, and compatible numbers.