Out of 100 numbers, 20 were 4s, 40 were 5s, 30 were 6s and the remaining were 7s. The mean of the numbers is :

1. 5.3

2. 5.4

3. 6.1

4. 6.5

If 36 a + 36 b = 576, then the mean of a and b is :

1. 6

2. 8

3. 12

4. 16

While computing the mean of grouped data, we assume that the frequencies are :

1. evenly distributed over all the classes

2. centred at the class marks of the classes

3. centred at the upper limits of the classes

4. centred at the lower limits of the classes

If the mean of five observations x, x + 2, x + 4, x + 6 and x + 8 is 11, then the mean of first three observations is :

1. 9

2. 11

3. 13

4. none of these