Calculate the simple interest on 18000 for 4 years at $12\dfrac{1}{2}$ % p.a.

Hint: In this question Simple interest is determined by multiplying the daily interest rate by the principal by the number of years that elapse between payments so use this concept to reach the solution of the question.

Complete step-by-step answer:

Given data
Principal amount (p) = 18000 Rs.
Rate of interest (r) = $12\dfrac{1}{2}$ % p.a.
Time (t) = 4 years.
Now as we know simple interest on any amount is calculated as
$ \Rightarrow S.I = \dfrac{{p.r.t}}{{100}}$.
So first simplify the rate of interest (i.e. convert the mixed fraction into improper fraction) we have,
$ \Rightarrow r = 12\dfrac{1}{2} = \dfrac{{\left( {12 \times 2} \right) + 1}}{2} = \dfrac{{25}}{2}$ %
Now put all these values in the formula of simple interest we have,
$ \Rightarrow S.I = \dfrac{{p.r.t}}{{100}} = \dfrac{{18000\left( {\dfrac{{25}}{2}} \right)\left( 4 \right)}}{{100}}$
Now simplify the above equation we have,
$ \Rightarrow S.I = \dfrac{{18000\left( {\dfrac{{25}}{2}} \right)\left( 4 \right)}}{{100}} = 180 \times 25 \times 2 = 9000$ Rs.
So, the simple interest is 9000 on 18000 for 4 years at $12\dfrac{1}{2}$ % p.a.
So, this is the required answer.

Note: In such types of questions the key concept we have to remember is that always recall the formula of simple interest which is stated above then substitute all the values in the formula as above and simplify we will get the required simple interest on 18000 for 4 years at $12\dfrac{1}{2}$ % p.a. which is the required answer.