There are 28 marbles in a blue jar. The blue jar has 4 times as many marbles as a red jar. Let n be the number of marbles in the red jar.

1. Break Down the Problem

We need to find the number of marbles $n$ in the red jar, given that the blue jar has 28 marbles and contains 4 times as many marbles as the red jar.

2. Relevant Concepts

According to the problem:

• Number of marbles in blue jar = 4 × Number of marbles in red jar
• Mathematically, this can be expressed as: $28 = 4n$

3. Analysis and Detail

To find the value of $n$, we can rearrange the equation: $n = \frac{28}{4}$

Calculating $n$: $n = 7$

4. Verify and Summarize

We can verify this by multiplying the number of marbles in the red jar by 4: $4n = 4 \times 7 = 28$ Thus, the calculations confirm our result.

Final Answer

The number of marbles in the red jar is $\boxed{7}$.