To find the mass of one tennis ball, you need to divide the total mass by the number of balls.
Given that the total mass of the three balls is 6 3/10 grams, you first convert the mixed number into an improper fraction:
6 3/10 = (6 * 10 + 3) / 10 = 63 / 10 grams.
Now, divide this by 3:
\[ \text{Mass of one ball} = \frac{63/10}{3} = \frac{63}{10} \times \frac{1}{3} = \frac{63}{30} = \frac{21}{10} \text{ grams}. \]
Now, convert it back to a mixed number:
\(\frac{21}{10} = 2 \frac{1}{10}\) grams.
Thus, the mass of one tennis ball is \(2 \frac{1}{10}\) grams or 2.1 grams.