Discussion
If 20% of an electricity bill is deducted, then Rs.100 is still to be paid. What is the amount of original bill ?
Answer: D
The original bill was Rs. 125.
If 20% is deducted, 80% of the bill remains, and that is Rs. 100:
\[ \begin{aligned} 0.8 \times \text{bill} &= 100 \\ \text{bill} &= 125 \end{aligned} \]
Check: 20% of 125 is 25, and 125 - 25 = 100. Rs. 120 comes from adding 20% to 100, which measures the deduction against the wrong amount.
In general, if x% is deducted and R is left to pay, the original amount is \(R \times \frac{100}{100 - x}\), here \(100 \times \frac{100}{80} = 125\).
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