If 20% of an electricity bill is deducted, then Rs. 100 is still to be paid. What is the original bill?

Appeared in: RML Lucknow - 2021

Explanation

The question states that after a 20% deduction, Rs. 100 is the remaining amount to be paid.
This means that the Rs. 100 represents the remaining 80% (100% - 20%) of the original bill.
To find the full (100%) original bill, you can set up the equation: 0.80 × Original Bill = 100.
Solving for the Original Bill gives: 100 / 0.80 = 125.
Therefore, the original bill amount was Rs. 125.

Option A: This is incorrect. It seems to be an arbitrary value close to 100, without a correct mathematical basis. It does not account for the 20% deduction properly.
Option B: This value is significantly higher than the correct answer. A possible error could be misinterpreting the percentage calculation entirely, perhaps by dividing 100 by a wrong decimal.
Option C: This is a common mistake where one might incorrectly calculate 20% of Rs. 100 (which is Rs. 20) and add it to 100, resulting in 120, and then round up or miscalculate to 130. The 20% deduction applies to the original unknown bill, not the final paid amount.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Calculating the original value based on a percentage decrease as background academic context rather than a clinical decision trigger.
While this is a mathematics question, the underlying skill of calculating percentages is crucial in many nursing tasks.
Nurses use percentage calculations for determining drug dosages, assessing weight loss or gain, calculating IV drip rates, and interpreting nutritional information.
Strong numeracy skills prevent medication errors and ensure patient safety.

How to Approach the Question

First, identify what the known values represent. Here, Rs. 100 is the amount after a deduction.
Determine the percentage that the known amount (Rs. 100) corresponds to. If 20% was deducted, the remaining part is 100% - 20% = 80%.
Set up a simple algebraic equation: 80% of 'X' (the original bill) equals 100, or 0.80 * X = 100.
Solve for 'X' by isolating it: X = 100 / 0.80.
Perform the division to find the final answer and check it against the given options.

Concept Tested & Keywords

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