The fourth proportional to 5, 8, 15 is :

1. 18

2. 20

3. 21

4. 24

The fourth proportional to 0.12, 0.21 and 8 is :

1. 8.9

2. 14

3. 17

4. 56

The mean proportion between 9 and 16 is:

1. 7

2. 12

3. 25

4. 144