Gwen bought one mobile phone at 25% 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 mobile phone?

	Percent expressed as units	Percent expressed as %	Amount
Original price	100 u		?
Discount	- 25 u
Discounted price	75 u	100%	$900
GST		+ 10%
Amount to pay		110%	$990

Price of the mobile phone with 10% GST in percent
= 100% + 10%
= 110%

110% of the mobile phone with 10% GST = 990
1% of the mobile phone with 10% GST = 990 ÷ 110 = 9
100% of the mobile phone with 10% GST = 100 x 9 = 900

Price of the mobile phone at 25% discount in units
= 100 u - 25 u
= 75 u

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

75 u of the mobile phone without GST = 900
1 u of the mobile phone without GST = 900 ÷ 75 = 12
100 u of the mobile phone without GST = 100 x 12 = 1200

Original price of the mobile phone = $1200

Answer(s): $1200