Opal bought one hi-fi system at 10% discount off its original price. She paid a total of $990 for it, which was inclusive of 10% GST on the discounted price. What was the original price of the hi-fi system?

	Percent expressed as units	Percent expressed as %	Amount
Original price	100 u		?
Discount	- 10 u
Discounted price	90 u	100%	$900
GST		+ 10%
Amount to pay		110%	$990

Price of the hi-fi system with 10% GST in percent
= 100% + 10%
= 110%

110% of the hi-fi system with 10% GST = 990
1% of the hi-fi system with 10% GST = 990 ÷ 110 = 9
100% of the hi-fi system with 10% GST = 100 x 9 = 900

Price of the hi-fi system at 10% discount in units
= 100 u - 10 u
= 90 u

Discounted price = 100% of the price to pay for before adding GST.

90 u of the hi-fi system without GST = 900
1 u of the hi-fi system without GST = 900 ÷ 90 = 10
100 u of the hi-fi system without GST = 100 x 10 = 1000

Original price of the hi-fi system = $1000

Answer(s): $1000