Question

Half litre oil costs rupees 45 then find out cost of $3\dfrac{1}{2}$ litre oil ?

Answer 1

Hint: For this type of questions first find the cost of one litre and then find the required amount with cost of one then we will get the required cost. Use the cross multiplication method for easy simplification.

Complete step-by-step answer:
The objective of the problem is to find the cost of $3\dfrac{1}{2}$ litre oil.
It is given that the cost of half litre oil is rupees 45.
Now we have to find the cost of one litre oil.
Let the cost of one litre oil be rupees x.

Oil	Rupee
$\dfrac{1}{2}$	45
1	x

On doing cross multiplication that is multiplying x with half and 1 with 45and equating .we get
$
   \Rightarrow x\left( {\dfrac{1}{2}} \right) = 1 \times 45 \\
   \Rightarrow \dfrac{x}{2} = 45 \\
   \Rightarrow x = 45 \times 2 \\
   \therefore x = 90 \\
$
Therefore, the cost of one litre oil is 90 rupees.
Now we have to find the cost of three and half litres of oil .
To find this we should multiply the cost of one litre oil with the required amount of oil .
$ = 3\dfrac{1}{2} \times 90$
Now converting the mixed fraction into normal fraction by multiplying three and two and adding one to the result obtained by multiplying. That is
$ = \dfrac{7}{2} \times 90$
Now on cancelling 90 with two tables we get the result 45.
$
   = 7 \times 45 \\
   = 315 \\
$
Therefore, the cost of three and half litre oil is rupees 315.

Note: The total amount of a complete work is always one. It is always necessary to find one unit for simple calculation. To find the unit price we divide the price of a certain number of units by the total number of units. Cross multiplication can be used to add or subtract unlike fractions easily.