Find the missing variable from the basic interest formula.
a. Ordinary Interest method
PRINCIPAL RATE TIME INTEREST
1 ₱ 8,000 7% 30 DAYS
2 ₱1,000 90 DAYS ₱50

To find the missing variable using the basic interest formula, we use:

\[
I = P \times R \times T
\]

Where:
- \( I \) is the interest
- \( P \) is the principal
- \( R \) is the rate (expressed as a decimal)
- \( T \) is the time (in years)

**1. Ordinary Interest Method** (using 360 days in a year):

### Problem 1:
- Principal (\(P\)) = ₱8,000
- Rate (\(R\)) = 7% or 0.07
- Time (\(T\)) = 30 days

Convert time into years:

\[
T = \frac{30}{360} = \frac{1}{12} \text{ years}
\]

Now, calculate the interest (\(I\)):

\[
I = 8,000 \times 0.07 \times \frac{1}{12}
\]

\[
I = 8,000 \times 0.07 \times 0.0833
\]

\[
I = ₱46.67
\]

**Interest (I) = ₱46.67**

### Problem 2:
- Principal (\(P\)) = ₱1,000
- Interest (\(I\)) = ₱50
- Time (\(T\)) = 90 days

Convert time into years:

\[
T = \frac{90}{360} = \frac{1}{4} \text{ years}
\]

Now, solve for the rate (\(R\)):

\[
I = P \times R \times T
\]

\[
50 = 1,000 \times R \times \frac{1}{4}
\]

\[
50 = 250 \times R
\]

\[
R = \frac{50}{250}
\]

\[
R = 0.2 \text{ or } 20\%
\]

**Rate (R) = 20%**

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Would you like further details or have any questions?

Here are some related questions:
1. How do you calculate interest using the Exact Interest method?
2. What is the effect of different compounding periods on the interest calculation?
3. How can you calculate the principal if the interest, rate, and time are known?
4. What is the difference between simple and compound interest?
5. How does changing the rate affect the total interest earned?

**Tip:** Always make sure to convert the time into years when using interest formulas, especially when the time is given in days or months.

Find the missing variable from the basic interest formula.
PRINCIPAL RATE TIME INTEREST
1 ₱ 8,000 7% 30 DAYS
2 ₱1,000 90 DAYS ₱50

Solve interest problems using the basic interest formula. Learn how to calculate missing interest and rate using the Ordinary Interest Method with real-life examples, such as calculating interest for ₱8,000 at 7% over 30 days and solving for the rate when ₱50 interest is earned over 90 days. Suitable for Grades 8-10.

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