Question

find a single discount equivalent to two successive discount of 20%and 5%

Answer 1

The formula here is -D1 - D2 + {(-D1) x (-D2)}/100
Equivalent discount = -20 - 5 + {100/100}
= -25 + 1 = -24%
Eg: Long Method
CP= ₹100.
D1 = 20%
D2= 5%
Now,
100- (20% of 100) = 100 - 20 = ₹80 (after D1)
80 - (5% of 80) = 80 - 4 = ₹76 (after D2)
Final SP is ₹76.
OR
Equivalent discount formula   100- (24% of 100) = 100 - 24 = ₹76

Answer 2

Let the amount/price = 100.
First discount = 20% :  20
So net price/cost =  80

next discount: 5% =  80*5/100 = 4
So final cost/price = 80-4 = 76

Single discount equivalent :  100 - 76 = 24 %

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Formula:
    Equivalent discount = d1 + d2 - (d1 * d2/100)
                                      = 20 + 5 - (20*5/100) =   24 %