Find the mean of marks obtained by 20 students in a class test (out of 50). 25, 40, 15, 16, 28, 39, 41, 22, 28, 30, 18, 22, 32, 48, 32, 38, 40.
A.30
B.31
C.32
D.33

Hint: First we will use a formula to calculate mean by adding up all the numbers and then divide by how many numbers there are, that is, 20. Then we will simplify it to find the required mean.

Complete step-by-step answer:
We are given that the marks obtained by 20 students are 25, 40, 15, 16, 28, 39, 41, 22, 28, 30, 18, 22, 32, 48, 32, 38, 40.
Then we know that the total number of students is 20.
 We know that the formula to calculate the mean is by adding up all the numbers and then dividing them by total numbers.
Here, we will calculate the mean by using the formula \[\dfrac{{{\text{sum of the marks obtained}}}}{{{\text{number of students}}}}\].
First, we will add the marks obtained by students, we get
\[
   \Rightarrow 25 + 40 + 15 + 16 + 28 + 39 + 41 + 22 + 28 + 30 + 18 + 22 + 32 + 48 + 32 + 38 + 40 \\
   \Rightarrow 514 \\
 \]
Dividing the above value by 20 to find the mean of marks obtained by 20 marks, we get
\[
   \Rightarrow \dfrac{{514}}{{20}} \\
   \Rightarrow 25.7 \\
 \]
Therefore, the mean of marks obtained by 20 students in a class test out of 50 is 25.7.
Hence, option B is correct.

Note: We need to know that the mean is adding the average of the numbers. It is easy to calculate: add up all the numbers, then divide by how many numbers there are. In other words it is the sum divided by the count. Do not forget any marks by adding up the values.