Jane bought one mobile phone at 20% discount off its original price. She paid a total of $872 for it, which was inclusive of 9% GST on the discounted price. What was the original price of the mobile phone?

	Percent expressed as units	Percent expressed as %	Amount
Original price	100 u		?
Discount	- 20 u
Discounted price	80 u	100%	$800
GST		+ 9%
Amount to pay		109%	$872

Price of the mobile phone with 9% GST in percent
= 100% + 9%
= 109%

109% of the mobile phone with 9% GST = 872
1% of the mobile phone with 9% GST = 872 ÷ 109 = 8
100% of the mobile phone with 9% GST = 100 x 8 = 800

Price of the mobile phone 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 mobile phone without GST = 800
1 u of the mobile phone without GST = 800 ÷ 80 = 10
100 u of the mobile phone without GST = 100 x 10 = 1000

Original price of the mobile phone = $1000

Answer(s): $1000