Question

The electricity bill of a certain establishment is partly fixed and partly varies with the number of units of electricity consumed. In a certain month when 540 units are consumed, the bill is Rs. 1,800. In another month the consumed units are 620 and the bill is Rs. 2,040. If 500 units of electricity is consumed in a month, the bill for that month is
1. Rs. 1,560
2. Rs. 1,600
3. Rs. 1,620
4. Rs. 1,680

Answer 1

Correct Answer - Option 4 : Rs. 1,680

Given:

For 540 units, the bill is Rs. 1,800

For 620 units, the bill is Rs. 2,040

Calculation:

Let the fixed amount be Rs. x

The cost of each unit be Rs. y.

Then,

⇒ x + 540y = 1,800

⇒ x + 620y = 2,040

Subtracting above results, we get

⇒ 80y = 240

⇒ y = 3 

Putting the above value in result (I)

⇒ x + 540(3) = 1,800

⇒ x + 1,620 = 1,800

⇒ x = 180

Now for 500 units, x + 500y 

⇒ 180 + 500(3)

⇒ 180 + 1,500

⇒ 1,680

∴ The bill for that month is Rs. 1,680.