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Percentage -Reciprocal Equivalence- Faster calculation practice!

QNR 4. Indian electricity prices have risen from Rs. 6.4 to Rs.6.5 per unit. Arvi’s factory expected to use 600 units in the next billing period. He installed solar panels at the start of that period. This reduced his electricity usage by 90%. How much did Arvi’s bill increase ?

1) Rs. 6                  

2) Rs.38                           

3) Rs.39                               

4) Rs. 54

Solution: The price has risen from Rs. 6.4 to Rs.6.5 per unit i.e. the price has risen by 0.1 per unit. The number of units is 10% of the expected units. [90% reduction of the original usage = 10% of the original usage] Number of units= 10% of 600 = 0.1 * 600 =60 Arvi’s bill increases by 60 units * 0.1 increase per unit. i.e. Rs. 6. Option (1).

Another variant of the question could be:- How much would be the increase in Arvi’s bill if electricity usage reduces by 83.33%, all other conditions remain same?

The level of the question increases by 1 level as the calculation gets bit time-taking to find 83.33% reduction on the expected usage: Number of units= 600 – 83.33% of 600 In place of the usual multiplication, it helps to use the reciprocal equivalent of 83.33% i.e 10/12. Number of units = 600 – 600 * (10/12) = 600 * (2/12) = 100. The price increases by 0.1 * 100 = Rs. 10.

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