A frequency distribution table shows marks obtained by students. The first quartile (Q1) is 35. Find the first quartile class and the value of x.

Since $20 < 35 < 40$, the first quartile class is 20-40.

The first quartile class is $\boxed{\text{20-40}}$.

From part (a), the first quartile class is 20-40.
$L = 20$ (lower boundary of the quartile class)
$f = x$ (frequency of the quartile class)
$cf = 2$ (cumulative frequency of the class preceding the quartile class)
$h = 40 - 20 = 20$ (class width)
$Q_1 = 35$ (given)
$N = 16 + x$ (total number of students)

Divide both sides by 5:

The value of $x$ is $\boxed{4}$.

The modal class is given as 40-60.
$L = 40$ (lower boundary of the modal class)
$f_m = 8$ (frequency of the modal class)
$f_1 = 4$ (frequency of the class preceding the modal class, 20-40)
$f_2 = 5$ (frequency of the class succeeding the modal class, 60-80)
$h = 60 - 40 = 20$ (class width)

The value of the mode is $\boxed{51.43}$.

From part (a), the first quartile class is 20-40.

Students below the 20-40 class are those in the 0-20 class.
Number of students below the first quartile class = 2.

Students above the 20-40 class are those in the 40-60, 60-80, and 80-100 classes.
Number of students above the first quartile class = $8 + 5 + 1 = 14$.

Ratio = (Number of students above) : (Number of students below)
Ratio = $14 : 2$

Divide both sides by 2:
Ratio = $7 : 1$

The ratio of students who are above and below the first quartile class is $\boxed{7:1}$.