Lower class limit of 15-18 is :

1. 15

2. 18

3. $\dfrac{18 - 15}{2}$

4. $\dfrac{15 + 18}{2}$

If the upper and the lower limits of a class interval are 16 and 10, the class-mark is :

1. 6

2. 3

3. 13

4. none of these

If the lower and the upper limits of a class interval are 7.5 and 12.5, the class-mark is :

1. 10

2. 2.5

3. 7.5

4. 12.5

In a pie-chart, an angle of 30° represents 80 articles. The number of articles represented by 105° are :

1. 280

2. $22\dfrac{6}{7}$

3. 200

4. none of these