The discount series 10%, 20%, 40% is equivalent to a single

Single equivalent discount for 10% and 20%

= 10 + 20 −	20 × 10	= 28%
	100

Single equivalent discount for 28% and 40%

=		40 + 28 −	28 × 40		%
			100

= (68 – 11.2) % = 56.8%
Alternate method to solve this question :
Here, D₁ = 10%, D₂ = 20%, D₃ = 40%

Single discount = 100 −

100 − D₁

100 − D₂

100 − D₃

× 100

100

Single discount = 100 −

100 − 10

100 − 20

100 − 40

× 100

100

Single discount = 100 −	90	×	80	×	60	× 100
	100		100		100

Single discount = 100 – 43.20 = 56.8%