The usual price of a pen sold at Shop S and T is $20. During a sale, a discount of 25% is given to every pen sold at Shop S. At Shop T, a discount of 40% is given to the 4th pen for every 4 pens sold. How much cheaper would it be if Min buys 11 pens from Shop S instead of Shop T?

	Shop S	Shop T
Price of 1 pen	75%  $15	100% $20
Every 4th pen		60% $12

Price of 1 pen from Shop S in percentage
= 100% - 25%
= 75%

Price of 1 pen from Shop S at 25% discount
= 75% x 20
= ⁷⁵₁₀₀ x 20 
= $15

Cost of 11 pens from Shop S
= 11 x 15
= $165

Price of the 4th pen from Shop T in percentage
= 100% - 40%
= 60%

Cost of 4th pen at Shop T
= 60% x 20
= ⁶⁰₁₀₀ x 20 
= $12

Cost of 1 set of 4 pens at Shop T
= 20 x 3 + 12
= $72

Number of sets of 4 for pens at Shop T
= 11 ÷ 4
= 2 r 3

Cost of 2 sets of 4 pens at Shop T
= 2 x 72
= $144

Cost of 3 pens at Shop T
= 3 x 12
= $36

Cost of 11 pens at Shop T
= 144 + 36
= $180

Amount cheaper if Min buys 11 pens from Shop S instead of Shop T
= 180 - 165
= $15

Answer(s): $15