To analyze the data provided in your table for April 2021, we need to summarize the sequence and calculate the required statistics.

Given the data:

| Số thứ tự | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|-----------|---|---|---|---|---|---|---|
| Số ngày | 4 | 10| 7 | 5 | 3 | 3 | 30|

### a) Mối của mẫu số liệu (Mean)
The mean (M) can be calculated using the formula:

\[
M = \frac{\sum (x_i \cdot f_i)}{\sum f}
\]

Where:
- \(x_i\) = value of each class (number of order)
- \(f_i\) = frequency (number of days)

Calculation:
\[
M = \frac{(0 \cdot 4) + (1 \cdot 10) + (2 \cdot 7) + (3 \cdot 5) + (4 \cdot 3) + (5 \cdot 3) + (6 \cdot 30)}{0 + 10 + 7 + 5 + 3 + 3 + 30}
\]
\[
M = \frac{0 + 10 + 14 + 15 + 12 + 15 + 180}{68} = \frac{236}{68} \approx 3.47
\]

### b) Số trung bình (Median)
To find the median (M_e), we need to arrange the data in ascending order and find the middle value.

The cumulative frequencies will help:

| Số thứ tự | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|-----------|---|---|---|---|---|---|---|
| Số ngày | 4 | 10| 7 | 5 | 3 | 3 | 30|
| Cumulative| 4 | 14| 21| 26| 29| 32| 62|

The median is located at position \( \frac{68}{2} = 34 \) in the cumulative frequency.

Looking at the cumulative frequency:
- 0: 4
- 1: 14
- 2: 21
- 3: 26
- 4: 29
- 5: 32
- 6: 62

The median falls in the cumulative frequency of 6.

### c) Số trung vị (Mode)
The mode (M_l) is the value that appears most frequently. Here the highest frequency is 10 for the value 1.

### d) Khoảng tứ phân vị (Quartiles)
Q1 and Q3 can be found using the cumulative frequencies:

- **Q1** is at position \( \frac{n}{4} = \frac{68}{4} = 17 \):
- Falls in the second interval (Số thứ tự = 1).

- **Q3** is at position \( \frac{3n}{4} = \frac{3 \times 68}{4} = 51 \):
- Falls in the sixth interval (Số thứ tự = 6).

### Summary of results:
- a) M (Mean) ≈ 3.47
- b) M_e (Median) = 6
- c) M_l (Mode) = 1
- d) Q1 = 1, Q3 = 6

Let me know if you need further assistance!