If an electricity bill is paid before due date, one gets a reduction of 4 % on the amount of the bill . By paying the bill before due date, a person got a reduction of ? 13 . The amount of his electricity bill was

Let the original price of be N .
? At discount of 20% article costs ? 596
? 596 = (80 x N)/100
? N = (596 x 100)/80
? N = 745
? original price = ? 745

Given, r₁ = 20% and r₂ = 20%
? Single discount equal to r₁ and r₂
= (r₁ + r₂ - r₁ x r₂) / 100 %
= (20 + 20 - 20 x 20) / 100
= 40 - 4 = 36%

Here, r₁ = 10 %, r₂ = 20% and r₃ = 30%
? Required discount
= [1 - (1 - r₁/100) (1 - r₂/100) (1 - r₃/100)] x 100%
= [1 - (1 -10/100) (1 - 20/100) (1 - 30/100)] x 100%
= {1 - (9/10) x (4/5) x (7/10)} x 100%
= (1 - 0.504) x 100% = 49.6%

Let the marked price of item = ? N
? {(100 - 14) /100 } of N = 645
? N = (645 x 100)/86 = ? 750

? Marked price of a shirt = ? 320
Discount on a shirt = 320 x 25/100 = ? 80

? Number of shirts has to be purchased to get a rebate of ? 400 = 400/80 = 5

? Single discount equivalent to 36% and 4%
= 36 + 4 - {(36 x 4)/100} = 38.56

? Required difference
= {(40 - 38.56) x 500}/100 = 1.44 x 5 = ? 7.20

Here, r = 20% and r₁ = 15%
Required profit per cent =[ r(100 - r₁)/100] - r₁
= [20(100 - 15)/100] - 15
= 17 - 15
= 2%