Question

: What is the cost of $3$ quintals $20$ kg of coal, if $3$ quintals cost Rs.$3900$?

Hint: We are given the cost of 3 quintals which is Rs. 3900. Find the cost of $1$quintal. So we know $1$quintal$=100$kg . So you have got the cost for $100$kg. Then find the cost for $20$kg. Try it and you will get the answer.

Mass is both a property of a physical body and a measure of its resistance to acceleration (a change in its state of motion) when a net force is applied. An object's mass also determines the strength of its gravitational attraction to other bodies.
The basic SI unit of mass is the kilogram (kg).
In everyday usage, mass and "weight" are often used interchangeably. For instance, a person's weight may be stated as $75$kg. In a constant gravitational field, the weight of an object is proportional to its mass, and it is unproblematic to use the same unit for both concepts.
But because of slight differences in the strength of the Earth's gravitational field at different places, the distinction becomes important for measurements with a precision better than a few percent, and places far from the surface of the Earth, such as in space or on other planets. Conceptually, "mass" (measured in kilograms) refers to an intrinsic property of an object, whereas "weight" (measured in newtons) measures an object's resistance to deviating from its natural course of free fall, which can be influenced by the nearby gravitational field. No matter how strong the gravitational field, objects in free fall are weightless, though they still have mass.
The quintal or center is a historical unit of mass in many countries which is usually defined as $100$ base units, such as pounds or kilograms. It is a traditional unit of weight in France, Portugal, and Spain and their former colonies. It is commonly used for grain prices in wholesale markets in India, where $1$quintal$=100$kg.

It is given in the question that the cost of $3$quintals is Rs $3900$.
It is asked to find the cost of $3$ quintals $20$kg.
We know $1$quintal$=100$kg.
So the cost for $1$ a quintal $=Rs.\dfrac{3900}{3}=Rs.1300$
The cost for $1$ quintal is Rs.$1300$.
Here $100$kg costs Rs$1300$.
So, $1$kg of coal costs$=Rs.\dfrac{1300}{100}=Rs.13$
So, $20$kg of coal costs$=Rs.20\times 13=$Rs.$260$.
So for the cost of $3$ quintals $20$kg$=Rs.(1300\times 3+20\times 13)=Rs.(3900+260)=Rs.4160$.
Hence, the cost for $3$ quintals $20$kg for coal is Rs$4160$.

Note: Carefully read the question. You should know the conversion that $1$ quintal$=100$kg.
Donâ€™t make mistakes while conversion. Most students make the mistakes in conversion itself. Keep in mind first you have to find the cost for $1$kg and quintal.