Answer:
3
Step-by-step explanation:
3² = 9, so √9 = 3
4² = 16, so √16 = 4.
12 is closer to 9 than to 16, so √12 is closer to √9 than to √16.
√12 ≈ 3 (to the closest integer)
The confidence interval would be (10.44, 12.16). This means that if we take repeated samples, the true mean lies in 90% of these intervals.
To find the confidence interval, we use:

We first find the z-value associated with this. To do this:
Convert 90% to a decimal: 90% = 90/100 = 0.9
Subtract from 1: 1-0.9 = 0.1
Divide by 2: 0.1/2 = 0.05
Subtract from 1: 1-0.05 = 0.95
Using a z-table (http://www.z-table.com) we see that this is directly between two z-scores, 1.64 and 1.65; we will use 1.645:
You can divide 20 by 10 and you should get 2, or do 20 divided by 2 and get 10.