Question

$72\%$ of 25 students are good in mathematics. How many are not good at mathematics?

Hint: As we know that the total number of students is 25. And as the percentage of good students is known so we can calculate the percentage of students who are not good in mathematics using$(100 - good\% )$. Hence, we calculate the number of students who are not good in maths using the above number of percentage of those students who are not good in mathematics, by using the formula, $n\% of A = \dfrac{n}{{100}} \times A$

As we know that the percentage of good students is $72\%$ and in total there are 25 students.
So, the percentage students not good in mathematics are,
$= (100 - good\% ) \\ = (100 - 72)\% \\ = 28\% \\$
Hence, in order to calculate the number of students who are not good at maths, we need to calculate a total $28\%$of 25
students.
As, $n\% ofA = \dfrac{n}{{100}} \times A$, using this we get,
$\Rightarrow$ Number of students who are not good in maths$= \dfrac{{28}}{{100}} \times 25$
On simplification we get,
$\Rightarrow$Number of students who are not good in maths$= \dfrac{{28}}{4}$
On dividing 28 by 7 we get,
$\Rightarrow$Number of students who are not good in maths$= 7$
Hence, the total number of students not good in mathematics is 7.

Note: Using the concept of mathematics that if $x\% + y\% = 100\%$and so if the percentage of one is known we can calculate the quantity of another quantity.
As the percentage can be given by,
$\dfrac{{n(notgood)}}{{n(total)}} = \dfrac{x}{{100}}$
Hence, the total number of not good students in maths can also be calculated.
Thus, do the calculation properly without any mistake.