The usual price of a pen sold at Shop Y and Z is $20. During a sale, a discount of 15% is given to every pen sold at Shop Y. At Shop Z, a discount of 30% is given to the 4th pen for every 4 pens sold. How much cheaper would it be if Gwen buys 15 pens from Shop Y instead of Shop Z?

	Shop Y	Shop Z
Price of 1 pen	85%  $17	100% $20
Every 4th pen		70% $14

Price of 1 pen from Shop Y in percentage
= 100% - 15%
= 85%

Price of 1 pen from Shop Y at 15% discount
= 85% x 20
= ⁸⁵₁₀₀ x 20 
= $17

Cost of 15 pens from Shop Y
= 15 x 17
= $255

Price of the 4th pen from Shop Z in percentage
= 100% - 30%
= 70%

Cost of 4th pen at Shop Z
= 70% x 20
= ⁷⁰₁₀₀ x 20 
= $14

Cost of 1 set of 4 pens at Shop Z
= 20 x 3 + 14
= $74

Number of sets of 4 for pens at Shop Z
= 15 ÷ 4
= 3 r 3

Cost of 3 sets of 4 pens at Shop Z
= 3 x 74
= $222

Cost of 3 pens at Shop Z
= 3 x 14
= $42

Cost of 15 pens at Shop Z
= 222 + 42
= $264

Amount cheaper if Gwen buys 15 pens from Shop Y instead of Shop Z
= 264 - 255
= $9

Answer(s): $9