A class consists of some boys and some girls along with a single teacher; after one class, the teacher drinks 9 litres of water, each boy drinks 7 litres of water, and each girl drinks 4 litres of water. If a total of 42 litres of water is consumed, how many girls are there in the class?

Introduction / Context:
This question involves forming and solving a linear equation based on a word problem. We are told how much water the teacher, each boy, and each girl drink. The total amount of water consumed is known, and we are asked to find how many girls are in the class. It is a typical aptitude problem that blends simple algebra with reading comprehension.

Given Data / Assumptions:
- There is 1 teacher in the class.
- Teacher drinks 9 litres of water.
- Each boy drinks 7 litres of water.
- Each girl drinks 4 litres of water.
- Total water consumed by everyone together is 42 litres.
- Numbers of boys and girls are whole numbers (non negative integers).

Concept / Approach:
Let the number of boys be B and the number of girls be G. We then express total water consumption as the sum of water used by teacher, boys, and girls. This leads to a linear equation in terms of B and G. Because the answer choices specify possible values for G, we can test each option to see which yields a valid integer value for B that satisfies the equation.

Step-by-Step Solution:
Let number of boys = B and number of girls = G. Teacher drinks 9 litres, boys together drink 7B litres, girls together drink 4G litres. Total water equation: 9 + 7B + 4G = 42. Rearrange: 7B + 4G = 42 − 9 = 33. So 7B + 4G = 33. Test G from options: for G = 3, 7B + 4*3 = 33 gives 7B + 12 = 33. Then 7B = 21, so B = 3, which is an integer and valid. Other G values do not give integer B, so G must be 3.

Verification / Alternative check:
Substitute B = 3 and G = 3 back into the total water calculation. Teacher: 9 litres. Boys: 3 * 7 = 21 litres. Girls: 3 * 4 = 12 litres. Total = 9 + 21 + 12 = 42 litres, which matches the given total. Therefore, the solution is consistent.

Why Other Options Are Wrong:
If G = 8, then 7B + 4*8 = 33 gives 7B + 32 = 33, so B = 1/7, not an integer.
If G = 6, 7B + 24 = 33 gives B = 9/7, again not an integer.
If G = 5, 7B + 20 = 33 gives B = 13/7, which is invalid as number of boys must be a whole number.

Common Pitfalls:
Many learners forget that the teacher also consumes water and omit the 9 litres from the equation. Others set up the equation correctly but do not insist on integer solutions for B and G. Some simply guess values rather than testing systematically against the equation. Using the options smartly can make such problems much easier.

Final Answer:
The number of girls in the class is 3.