Worksheet for practicing simple and compound interest calculations with examples and formulas.

Let’s solve each problem step by step using the simple interest formula:

Simple Interest Formula:
> I = p × r × t
Where:
- I = interest earned or paid
- p = principal (starting amount)
- r = annual interest rate (as a decimal)
- t = time in years

We’ll convert months to years when needed:
→ 6 months = 0.5 years
→ 15 months = 15/12 = 1.25 years
→ 30 months = 30/12 = 2.5 years
→ 2½ years = 2.5 years

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Problem 1: $300 at 7.5% for 5 years

p = 300
r = 7.5% = 0.075
t = 5

I = 300 × 0.075 × 5
= 300 × 0.375
= 112.50

✔ Answer: $112.50

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Problem 2: $770 at 16% for 6 months

p = 770
r = 16% = 0.16
t = 6/12 = 0.5

I = 770 × 0.16 × 0.5
= 770 × 0.08
= 61.60

✔ Answer: $61.60

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Problem 3: $668 at 9.25% for 15 months

p = 668
r = 9.25% = 0.0925
t = 15/12 = 1.25

I = 668 × 0.0925 × 1.25

First, 668 × 0.0925:

668 × 0.09 = 60.12
668 × 0.0025 = 1.67
Total = 60.12 + 1.67 = 61.79

Now multiply by 1.25:

61.79 × 1.25 = ?

Break it down:
61.79 × 1 = 61.79
61.79 × 0.25 = 15.4475
Total = 61.79 + 15.4475 = 77.2375

Rounded to nearest cent → $77.24

✔ Answer: $77.24

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Problem 4: $285 at 8.5% for 2½ years

p = 285
r = 8.5% = 0.085
t = 2.5

I = 285 × 0.085 × 2.5

First, 285 × 0.085:

285 × 0.08 = 22.80
285 × 0.005 = 1.425
Total = 22.80 + 1.425 = 24.225

Now × 2.5:

24.225 × 2.5 = ?

24.225 × 2 = 48.45
24.225 × 0.5 = 12.1125
Total = 48.45 + 12.1125 = 60.5625

Rounded → $60.56

✔ Answer: $60.56

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Problem 5: $360 at 185% for 2 years

Wait — 185%? That seems extremely high, but we’ll go with it as written.

p = 360
r = 185% = 1.85
t = 2

I = 360 × 1.85 × 2

First, 360 × 1.85:

360 × 1 = 360
360 × 0.85 = 306
Total = 360 + 306 = 666

Then × 2 = 1332

✔ Answer: $1,332.00

*(Note: This is unusual — likely a typo meant to be 1.85%, but since the problem says 185%, we use that.)*

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Problem 6: $175 at 5.45% for 30 months

p = 175
r = 5.45% = 0.0545
t = 30/12 = 2.5

I = 175 × 0.0545 × 2.5

First, 175 × 0.0545:

175 × 0.05 = 8.75
175 × 0.0045 = 0.7875
Total = 8.75 + 0.7875 = 9.5375

Now × 2.5:

9.5375 × 2.5 = ?

9.5375 × 2 = 19.075
9.5375 × 0.5 = 4.76875
Total = 19.075 + 4.76875 = 23.84375

Rounded → $23.84

✔ Answer: $23.84

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Problem 7: Sarah invested $150 at 8% simple interest. What is total after 3 years?

This asks for total amount, not just interest.

So: Total = Principal + Interest

p = 150
r = 8% = 0.08
t = 3

Interest = 150 × 0.08 × 3 = 150 × 0.24 = 36

Total = 150 + 36 = $186

✔ Answer: $186.00

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Problem 8: Ben borrowed $800 at 9% simple interest for 2 years. Total to repay?

Again, total repayment = principal + interest

p = 800
r = 9% = 0.09
t = 2

Interest = 800 × 0.09 × 2 = 800 × 0.18 = 144

Total = 800 + 144 = $944

✔ Answer: $944.00

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Final Answer:
1. $112.50
2. $61.60
3. $77.24
4. $60.56
5. $1,332.00
6. $23.84
7. $186.00
8. $944.00