(i) Find the class mark of the class 90 – 120.

(i) class mark $=\frac{\text { upper limit+lower limit }}{2}=\frac{120+90}{2}=\frac{210}{2}=105$

(ii) mid-value = 10
width = 6
Let the lower limit of the class be x. 
upper limit = x + 6

class $\mathrm{mark} / \mathrm{mid}-\mathrm{value}=\frac{\text { upper limit+lower limit }}{2}$

$\frac{x+(x+6)}{2}=10$

$\Rightarrow x=7$

(iii) width = 5
lower class limit of lowest class = 10
The classes will be 10-15, 15-20, 20-25, 25-30, 30-35.
Upper class limit of the highest class = 35.
(iv) Class marks = 15, 20, 25, ...
class size = 20 -">−- 15 = 5
Let lower limit of class be x.

$\frac{x+(x+5)}{2}=20$

$\Rightarrow x=17.5$

Thus, the class is 17.5-22.5. 
(v) 20 will be included in the class interval 20-30.