Assertion (A) : The sum of two numbers is $\dfrac{28}{25}$ of the first number. Then the second number is 12% of the first number.

Reason (R) : To express one quantity as a percentage of the other we write the other quantity as fraction of the one and then multiply by 100.

1. Both A and R are correct, and R is the correct explanation for A.

2. Both A and R are correct, and R is not the correct explanation for A.

3. A is true, but R is false.

4. A is false, but R is true.

Let the first number be x and second number be y.

According to question,

$⇒ x + y = \dfrac{28x}{25} \\[1em] ⇒ y = \dfrac{28x}{25} - x \\[1em] ⇒ y = \dfrac{28x - 25x}{25} \\[1em] ⇒ y = \dfrac{3x}{25} \\[1em]$

Expressing y as the percentage of the first number :

$\Rightarrow \dfrac{y}{x} \times 100\% \\[1em] \Rightarrow \dfrac{\dfrac{3x}{25}}{x} \times 100\% \\[1em] \Rightarrow \dfrac{3}{25} \times 100\%\\[1em] \Rightarrow 3 \times 4\%\\[1em] \Rightarrow 12\%$

So, assertion (A) is true.

According to reason :

To express one quantity as a percentage of the other we write the other quantity as fraction of the first and then multiply by 100.

Lets take an example if we want to know what percent 30 is of 150,

$\Rightarrow \dfrac{30}{150} \times 100 \\[1em] \Rightarrow \dfrac{1}{5} \times 100 \\[1em] \Rightarrow 20\%.$

So, reason (R) is true and reason clearly explains assertion.

Hence, option 1 is the correct option.