\[72\% \] of \[25\] students are good in Mathematics. How many are not good in Mathematics?

Hint: To find the number of students not good in mathematics, find the percentage of students not good in mathematics and then we have to calculate percent of the number, divide the percentage by \[100\] and multiply by the total number of students.
\[x\% \] of a number \[ = \dfrac{x}{{100}} \times {\text{number}}\].

Complete step-by-step solution:
We are given this problem,
Total number of students \[ = 25\]
Students good in maths \[ = 72\% {\text{ of }}25\]
We already know that the total percentage is \[100\% \]. We know that \[72\% \] students are good at mathematics.
In order to determine the students not good in mathematics subtract \[72\% \] from total \[\% \].
Students not good in maths \[ = (100 - 72)\% \]
Students not good in maths \[ = 28\% \].
Now we have found out the percentage of students who are not good at mathematics = \[28\% \].
To find the number of students not good in mathematics, convert the percentage into numbers by “divide the percentage by \[100\] and multiply by total number of students (\[25\])”.
To find the percentage, \[x\% \] of number
Students not good in maths \[ = 28\% \]
\[\% \] can be removed by dividing \[x\] by \[100\] and “of” means multiplication.
Thus, \[x\% \] of numbers \[ = \dfrac{x}{{100}} \times {\text{number}}\].
 \[ = \dfrac{{28}}{{100}} \times 25\]
  \[ = \dfrac{{28}}{4}\]
On further simplification, then
Students not good in maths \[ = 7\]
Therefore, Number of students not good in mathematics is \[28\% = 7\].

Note: Remember that the percentage is always out of \[100\], i.e., the total of percentage is always \[100\% \].
\[x\% \],the percentage symbol can be removed by writing it in fraction. \[x\% = \dfrac{x}{{100}}\].
Example: \[42\% = \dfrac{{42}}{{100}}\].
To express \[\dfrac{a}{b}\]as a percent: we have, \[\dfrac{a}{b} = \left( {\dfrac{a}{b} \times 100} \right)\% \] .