A cardboard is 4.5 m long and 3.5 m wide. What is the cost of painting it on both sides at the rate of Rs. 1.25\[{{m}^{2}}\] per is?

Hint: To solve the question, we have to calculate the area of the given cardboard to calculate the portion of area to be painted. The obtained value when multiplied with the rate of cost of painting, will give us the total cost of painting the cardboard.

Complete step-by-step answer:
We know that the portion of area to be painted is equal to twice the area of the rectangular board.
We know the formula for the area of the rectangular board is equal to the product of length and breadth of the rectangle which is equal to \[lb\]
Where l, b are the length and breadth of the triangle.
The given length of the cardboard is equal to 4.5 m
The given breath of the cardboard is equal to 3.5 m
By substituting the values of the length and breadth of rectangle in the above-mentioned formula of area of a rectangular board, we get
\[A=4.5\times 3.5\]
Where A represents the area of a cardboard.
\[A=15.75{{m}^{2}}\]
Thus, the area of the rectangular cardboard of 4.5 m length and 3.5 m width equal to 15.75\[{{m}^{2}}\]
The cost of painting the rectangular board on the both sides at the rate of Rs. 1.25 per\[{{m}^{2}}\] = 1.25 \[\times \] 2(area of the triangular board of given sides)
Thus, the cost of painting the rectangular board on the both sides at the rate of Rs. 1.25 per\[{{m}^{2}}\] = \[1.25\times 2\times 15.75\] = Rs. 39.375

Note: The possibility of mistake can be, not being able to analyse the need to calculate the area of the rectangular board to calculate the total cost of painting of the rectangular board of given sides. The other possibility of mistake can be, analysing that the total cost of painting is calculated by multiplying the area of the cardboard and the rate of cost of painting per square metre, but the question mentions the cardboard is painted on both sides. Thus, the total area painted, which is twice the area of cardboard, should be multiplied with the rate of cost of painting per square metre, to calculate the answer.