John has a bag of marbles. The ratio of red marbles to blue marbles is 3 : 7. What percent of the marbles are red?

To find the percentage of red marbles in the bag, we can use the ratio provided, which is 3 red marbles to 7 blue marbles (a total of \(3 + 7 = 10\) marbles).

The percentage of red marbles is calculated as:

\[
\text{Percentage of red marbles} = \left( \frac{\text{Number of red marbles}}{\text{Total number of marbles}} \right) \times 100
\]

Substitute the values:

\[
\text{Percentage of red marbles} = \left( \frac{3}{10} \right) \times 100 = 30\%
\]

So, **30%** of the marbles are red.

Would you like further explanation or have any questions?

Here are 5 related questions to deepen understanding:
1. If John has 50 marbles in total, how many are red and how many are blue?
2. What is the ratio of blue marbles to total marbles?
3. If 60% of the marbles were blue, what would be the ratio of red to blue marbles?
4. If John adds 5 more red marbles to the bag, how does the ratio of red to blue marbles change?
5. How would you express the ratio of red to blue marbles as a fraction?

**Tip:** When working with ratios, always consider the total number of parts involved to make calculations easier.

Learn how to calculate the percentage of red marbles in a bag where the ratio of red marbles to blue marbles is 3:7. This problem covers basic concepts in ratios and percentages, suitable for students in Grades 5-7.

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