David and Billy went to buy stereo systems at a sale. The usual price of 1 stereo system was $2000 each.

SALE
Buy 1 stereo system at 5% discount
Buy 2 or more stereo systems at 10% discount each

1. David bought 1 stereo system. How much did he pay?
2. What is the maximum number of stereo systems Billy can buy if he has $7000?

(a)
Discounted price of 1 stereo system in percent if only 1 was bought
= 100% - 5%
= 95%

100% of 1 stereo system = 2000
1% of 1 stereo system = 2000 ÷ 100 = 20
95% of 1 stereo system = 95 x 20 = 1900 

Amount that David paid for 1 stereo system = $1900

(b)
Discounted price of 1 stereo system in percent if more than 1 was bought
= 100% - 10%
= 90%

90% of 1 stereo system = 90 x 20 = 1800

Amount that Billy paid for 1 stereo system = $1800

Maximum number of stereo systems that Billy can buy if he has $7000
= 7000 ÷ 1800
= 3 r 1600

Answer(s): (a) $1900; (b) 3