Half-Life Practice worksheet with educational questions and illustrations.

Let’s solve each part step by step.

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1. Calculating percentages

We are asked to find what percent of items are *not* a certain type — meaning we subtract the given number from the total, then divide by total and multiply by 100.

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a. Sara has 8 bags of skittles. Two are Tropical, three are sour. What percent are NOT sour?

Total bags = 8
Sour bags = 3
Not sour = 8 - 3 = 5

Percent not sour = (5 8) × 100 = ?

5 ÷ 8 = 0.625
0.625 × 100 = 62.5%

✔ Answer for 1a: 62.5%

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b. Joe has 15 bags of cheetos. 8 are regular, 7 are hot. What percent are NOT Doritos?

Wait — this is tricky. The problem says “cheetos”, but asks about “Doritos”. That seems like a trick or typo.

But let’s read carefully: It says Joe has 15 bags of *cheetos*. Then it says 8 are regular, 7 are hot. So all 15 are cheetos — none are Doritos.

So if none are Doritos, then ALL 15 are NOT Doritos.

Percent not Doritos = (15 ÷ 15) × 100 = 100%

Even though it mentions “hot” and “regular”, those are types of Cheetos — still not Doritos.

✔ Answer for 1b: 100%

*(Note: If this was meant to say “not hot”, then it would be 8/15 ≈ 53.3%. But as written, it says “not Doritos”, so we go with 100%.)*

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c. Josh found a rock with 200 carbon-14 atoms. After 14 days, 25 remain. What percent are still unstable?

Unstable atoms = the ones that haven’t decayed yet → 25 atoms

Total original = 200

Percent still unstable = (25 ÷ 200) × 100

25 ÷ 200 = 0.125
0.125 × 100 = 12.5%

✔ Answer for 1c: 12.5%

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2. Complete the half-life table

The table shows:

| # of Half-Lives | % Unstable |
|------------------|------------|
| 0 | |
| 1 | |
| 2 | |
| 3 | |
| 4 | |

Half-life means after each half-life, half of the remaining unstable atoms decay.

Start at 100% unstable at time 0.

After 1 half-life → 50% left
After 2 → 25% left
After 3 → 12.5% left
After 4 → 6.25% left

So fill in:

| # of Half-Lives | % Unstable |
|------------------|------------|
| 0 | 100% |
| 1 | 50% |
| 2 | 25% |
| 3 | 12.5% |
| 4 | 6.25% |

✔ Table completed.

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3. Kenton’s fossil

Fossil has 12.5% uranium-235 left.
Half-life of U-235 = 704 million years.

We need to find how many half-lives have passed to get to 12.5%.

From above table:

→ 0 half-lives: 100%
→ 1: 50%
→ 2: 25%
→ 3: 12.5% ← This matches!

So 3 half-lives have passed.

Each half-life = 704 million years

Total age = 3 × 704 = ?

3 × 700 = 2100
3 × 4 = 12
Total = 2100 + 12 = 2112 million years

Or 2.112 billion years — but since question says “how many years old”, and gives half-life in millions, we’ll keep it in millions.

✔ Answer: 2112 million years

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Final Answer:

1a. 62.5%
1b. 100%
1c. 12.5%

2. Table:
0 → 100%
1 → 50%
2 → 25%
3 → 12.5%
4 → 6.25%

3. 2112 million years