The single discount equal to three consecutive discounts

Single equivalent discount for 10% and 12%.

=		12 + 10 −	20 × 10		% = 20.8%
			100

Single equivalent discount for 20.8% and 5%.

=		20.8 + 5 −	20.8 × 5		% = 24.76%
			100

Alternate method to solve this question :
Here, D₁ = 10%, D₂ = 12%,D₃ = 5%
Single equivalent discount

= 100 −

100 − D₁

100 − D₂

100 − D₃

× 100

100

= 100 −

100 − 10

100 − 12

100 − 5

× 100

100

= 100 −	90	×	88	×	95	× 100
	100		100		100

= 100 – 75.24 = 24.76%