Final Answer:
1. $2,800
2. $2,222.22
3. $108
4a. Peter used 0.8 instead of 0.08 for the interest rate.
4b. $264
5a. Isabella added the interest to the principal twice (or misread the total).
5b. $720
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Explanation:
Let’s go problem by problem so you can see how each one works.
Problem 1:
Mrs. Baxter puts $2,000 in an account with 5% simple interest for 8 years.
Simple interest formula: I = p × r × t
I = interest, p = principal (starting money), r = rate (as a decimal), t = time in years.
First, turn 5% into a decimal → 5 ÷ 100 = 0.05
Now plug in: I = 2000 × 0.05 × 8 = 800
That’s the interest earned. But the question asks how much the investment is worth — that means principal + interest.
So: 2000 + 800 = $2,800
Problem 2:
Joey earned $400 in interest over 3 years at 6%. We need to find his starting deposit (p).
Formula: I = p × r × t → rearrange to solve for p: p = I ÷ (r × t)
Turn 6% into decimal → 0.06
p = 400 ÷ (0.06 × 3) = 400 ÷ 0.18 ≈ 2,222.22
So Joey’s initial deposit was about $2,222.22
Problem 3:
Coach Cliffman put $1,800 at 2% for 3 years. Find just the interest.
I = p × r × t
2% as decimal = 0.02
I = 1800 × 0.02 × 3 = 108
So he earned $108 in interest.
Problem 4:
Peter used I = 275 × 0.8 × 12 → but 8% is 0.08, not 0.8! That’s his mistake.
Correct: I = 275 × 0.08 × 12 = 264
So correct interest is $264.
Problem 5:
Isabella got I = 3000 × 0.04 × 6 = 720 → that’s correct interest.
But she said the total is $6,720 — that would mean she added the interest to the principal twice (3000 + 720 + 720 = 4440? No, wait — 3000 + 720 = 3720, not 6720). So she probably added wrong or thought the interest was $3,720. Her total should be 3000 + 720 = $3,720, not $6,720.
The correct interest is still $720.
You’re doing great — just watch those decimals and what the question is asking (interest only vs. total amount)!
Parent Tip: Review the logic above to help your child master the concept of i prt worksheet.
