Correct Option: C

Marked price = ₹ 720
Actual price = ₹ 550.80
First discount D₁ = 10 %
Let the second discount = D₂ %
As we know the formula,

Successive discounts of D1 % and D2 % is overall equals to =		10 + D2 −	10 × D2		%
			100

= 10 + D2 −	10 × D2
	100

= 10 + D2 −	D2
	10

= 10 + D₂ - 0.1D₂
= 10 + 0.9D₂
According to question,
720 x ( 100 - 10 - 0.9D₂) % = 550.80
⇒  720 x ( 90 - 0.9D₂) = 550.80 x 100

⇒  90 - 0.9D2 =	55080
	720

⇒  90 - 0.9D2 =	5508
	72

⇒  90 - 0.9D2 =	612
	8

⇒  0.9D2 = 90 -	612
	8

⇒  0.9D2 =	720 - 612
	8

⇒  0.9D2 =	108
	8

⇒  0.9D2 =	27
	2

⇒  D2 =	27	× 0.9
	2

⇒  D2 = 27 ×	10	× 9
	2

⇒  D2 = 3 ×	10	=	30
	2		2

∴  Second discount = 15%

2nd Method to solve this question.
Here, Marked price = ₹ 720
Selling price = ₹ 550.80
First discount D₁ = 10 %
Let assume the second discount = D₂ %
As we know the formula,

Selling Price = Market Price		100 − D1			100 − D2
		100			100

550.80 = 720		100 − 10			100 − D2
		100			100

⇒  550.80 = 720 ×	90	×	100 - D2
	100		100

⇒  550.80 = 72 ×	9	×	100 - D2
	1		100

⇒  550.80 = 72 × 9 x	(100 - D2)
	100

⇒  550.80 x 100 = 72 × 9 x ( 100 - D₂ )
⇒  55080 = 648 x ( 100 - D₂ )
⇒  55080 = 648 x 100 - 648 x D₂
⇒  55080 = 64800 - 648 x D₂
⇒  648 x D₂ = 64800 - 55080
⇒  648 x D₂ = 9720

⇒  D2 =	9720
	648

⇒  D₂ = 15 %