Hello, Mukilab!
I'm not sure if I understand the problem,
. . but I'll take a guess . . .
I understand how histograms work and such but I just don't understand this one.
Table:
$\displaystyle \begin{array}{ccc}\text{Time} & \text{Frequency} \\ \hline
0<t \leq 10 & 20 \\
10<t \leq15 & \text{blank} \\
15 < t \leq 17.5 & 30 \\
17.5 <t \leq20 & 40 \\
20<t\leq25 & \text{blank} \\
25<t \leq40 & \text{blank} \end{array}$
Complete the histogram. I see no way to fill in the blanks, so I must assume that "blank" means zero frequency.
The frequency represents an area above the interval.
The first interval is [0,10], ten units wide.
The area is 20.
Hence, the height is 2.
The third interval is [15, 17.5], 2½ units wide.
The area is 30.
Hence, the height is: .$\displaystyle 30 \div 2\tfrac{1}{2} \,=\,12$
The fourth interval is [17.5, 20], 2½ units wide.
The area is 40.
Hence, the height is: .$\displaystyle 40 \div 2\tfrac{1}{2} \:=\:16$
The histogram looks something like this: Code:

 16
 **
 12::
 **::
 ::::
 ::::
 2 ::::
** ::::
::::::::::: ::::
 +   +   +   + + +   +   +   +   +  
0 5 10 15 20 25 30 35 40