Question

In a survey music preferences

Answer 1

81/450 is 0.18.
If you move the decimal place to the right two spots, you've got your percentage.
18% said they preferred country music.

Answer 2

We can write all information we have as:

81 answers = x% of 450 students

So we just need to do a simple equation:

81 = x% * 450
81 = $\frac{x}{100}$ * 450
81 = $\frac{x}{10}$ * 45
81 = $\frac{45x}{10}$                 / * 10 (both sides)
810 = 45x                    / ÷ 45 (both sides)
x = 18 [percent]

Answer: 18% of the students preferred country music.