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.