Directions: Read the following frequency distribution for two series of observations and answer the two items that follow:
| Class interval | Series-I | Series-II |
|---|---|---|
| 10–20 | 20 | 4 |
| 20–30 | 15 | 8 |
| 30–40 | 10 | 4 |
| 40–50 | X | 2X |
| 50–60 | Y | Y |
| Total | 100 | 100 |
What is the mean of frequency distribution of Series-I?