The single discount equivalent to the discount series of

Single equivalent discount for 20% and 10%

=		20 + 10 −	20 × 10		% = 28%
			100

Single equivalent discount for 28% and 5%

=		28 + 5 −	28 × 5		% = 31.6%
			100

Alternate method to solve this question :
Here, D₁ = 20%, D₂ = 10%,D₃ = 5%
Single equivalent discount

= 100 −

100 − D₁

100 − D₂

100 − D₃

× 100

100

= 100 −

100 − 20

100 − 10

100 − 5

× 100

100

= 100 −	80	×	90	×	95	× 100
	100		100		100

= 100 – 68.40 = 31.60%