Hilda bought one hi-fi system at 20% discount off its original price. She paid a total of $428 for it, which was inclusive of 7% 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	- 20 u
Discounted price	80 u	100%	$400
GST		+ 7%
Amount to pay		107%	$428

Price of the hi-fi system with 7% GST in percent
= 100% + 7%
= 107%

107% of the hi-fi system with 7% GST = 428
1% of the hi-fi system with 7% GST = 428 ÷ 107 = 4
100% of the hi-fi system with 7% GST = 100 x 4 = 400

Price of the hi-fi system at 20% discount in units
= 100 u - 20 u
= 80 u

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

80 u of the hi-fi system without GST = 400
1 u of the hi-fi system without GST = 400 ÷ 80 = 5
100 u of the hi-fi system without GST = 100 x 5 = 500

Original price of the hi-fi system = $500

Answer(s): $500