Warm-Up 1
What is 50% of 80?
Vocabulary: A
benchmark percent is a common percent that
is easy to compute mentally. 50% means half. To find 50% of a number,
divide it by 2.
✓Correct! 50% of 80 = 80 ÷ 2 = 40.
✗50% means half. 80 ÷ 2 = 40 (answer B).
Warm-Up 2
What is 10% of 200?
Hint: To find 10% of a number, divide by 10. This is
a key benchmark percent to memorize.
10% of 200 =
✓Correct! 10% of 200 = 200 ÷ 10 = 20.
✗10% means divide by 10. 200 ÷ 10 = 20.
Practice 1
Estimate 25% of 84. Which answer is the best
estimate?
Strategy: 25% = one quarter. To
estimate, round 84 to 80 first. 25% of 80 =
80 ÷ 4 = 20. Is your answer reasonable?
Show your estimation strategy:
Sentence frame: "I rounded 84 to ___. 25% of ___ = ___ ÷ 4 = ___. So
25% of 84 is about ___."
✓Correct! 25% of 84 ≈ 25% of 80 = 20. (Exact: 21.)
✗25% = 1/4. Round 84 to 80. 80 ÷ 4 = 20 (answer
B).
Practice 2
Estimate 10% of 53, then use it to estimate 20% of 53.
Strategy: 10% of 53 ≈ 10% of 50 = 5. To find 20%,
double the 10% amount: 5 × 2 = 10.
Benchmark percents build on each other!
Sentence frame: "10% of 53 ≈ ___. Since 20% is double 10%, 20% of 53
≈ ___ × 2 = ___."
✓Correct! 10% of 53 ≈ 5 and 20% ≈ 10.
✗10% of 53 ≈ 5 (round to 50, divide by 10). 20% ≈ 5
× 2 = 10.
Practice 3
Match each benchmark percent calculation to the correct estimate.
Remember: 10% = ÷ 10. 25% = ÷ 4. 50% = ÷ 2. Use these
benchmark percents with
reasonable rounding.
✓All correct! 10% of 60 = 6, 50% of 60 = 30, 25% of 60 = 15, 25% of
100 = 25.
✗Some matches are wrong. 10% of 60 = 6, 50% of 60 = 30, 25% of 60 =
15, 25% of 100 = 25.
Practice 4
A jacket costs $78. It is on sale for 50% off. Which is the best
estimate of the discount?
Strategy: 50% off means half off. Round $78 to $80.
Half of $80 = $40. Is your estimate
reasonable?
Sentence frame: "50% means half. I rounded $78 to $___. Half of $___
= $___. The discount is about $___."
✓Correct! 50% of $78 ≈ 50% of $80 = $40.
✗50% means half. Round $78 → $80. $80 ÷ 2 =
$40 (answer C).
Practice 5
Kenji says 10% of 45 is 45. Is his answer reasonable?
Think about it: 10% is a small part -- just
one-tenth. If 10% of 45 = 45, that would mean 10% = 100%. That is not
reasonable. What is 45 ÷ 10?
Sentence frame: "10% of 45 should be ___ of 45, which is ___.
Kenji's answer of 45 is not reasonable because ___."
✓Correct! 10% of 45 = 4.5, not 45. Kenji's answer is not
reasonable.
✗10% of 45 = 45 ÷ 10 = 4.5. Saying 10% of 45 = 45 is
not reasonable.
Challenge
Estimate 15% of 82.
Hint: Break 15% into benchmark percents you know.
Strategy: 15% = 10% + 5%. Find 10% of 82 first (≈ 8),
then 5% is half of 10% (≈ 4). Add them: 8 + 4 = 12.
Show how you broke 15% into benchmarks:
Sentence frame: "15% = 10% + 5%. 10% of 82 ≈ ___. 5% of 82 ≈ ___. So
15% of 82 ≈ ___ + ___ = ___."
✓Correct! 10% of 82 ≈ 8, 5% ≈ 4. So 15% ≈ 12. (Exact: 12.3.)
✗15% = 10% + 5%. 10% of 82 ≈ 8. 5% ≈ 4. Total ≈
12 (answer C).