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Question

The electricity bill of a certain establishment is partly fixed and partly variable depending on the number of units of electricity consumed. In a certain month when 540 units were consumed, the bill was ₹1,800. In another month when 620 units were consumed, the bill was ₹2,040. If in a month 500 units are consumed, then the bill for that month will be:

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
₹1,680

The problem involves calculating an electricity bill composed of a fixed charge and a variable charge based on electricity units consumed. We need to determine the fixed and variable rates from the given data and then calculate the bill for a specific consumption.

Formulating Equations for Electricity Bill

Let F represent the fixed charge (in ₹) and V represent the variable charge per unit (in ₹/unit). The total bill (B) is calculated using the formula: B = F + V × U, where U is the number of units consumed.

From the given information:

  • Month 1: 540 units consumed, Bill = ₹1,800. This gives the equation:
    $1800 = F + 540V$ (Equation 1)
  • Month 2: 620 units consumed, Bill = ₹2,040. This gives the equation:
    $2040 = F + 620V$ (Equation 2)

Calculating Variable Charge per Unit (V)

To find the variable charge per unit (V), we subtract Equation 1 from Equation 2:

$(2040 - 1800) = (F + 620V) - (F + 540V)$
$240 = 620V - 540V$
$240 = 80V$

Now, solve for V:

$V = \frac{240}{80}$
$V = 3$

So, the variable charge is ₹3 per unit.

Calculating Fixed Charge (F)

Substitute the value of V (₹3) back into Equation 1 to find the fixed charge (F):

$1800 = F + 540 \times 3$
$1800 = F + 1620$
$F = 1800 - 1620$
$F = 180$

The fixed charge is ₹180.

Calculating Bill for 500 Units

Now, calculate the bill for a month when 500 units are consumed using the determined values of F and V:

$B = F + V \times U$
$B = 180 + 3 \times 500$
$B = 180 + 1500$
$B = 1680$

Therefore, the bill for 500 units consumed will be ₹1,680.

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