The usual price of a pen sold at Shop M and N is $30. During a sale, a discount of 20% is given to every pen sold at Shop M. At Shop N, a discount of 30% is given to the 6th pen for every 6 pens sold. How much cheaper would it be if Penelope buys 15 pens from Shop M instead of Shop N?

	Shop M	Shop N
Price of 1 pen	80%  $24	100% $30
Every 6th pen		70% $21

Price of 1 pen from Shop M in percentage
= 100% - 20%
= 80%

Price of 1 pen from Shop M at 20% discount
= 80% x 30
= ⁸⁰₁₀₀ x 30 
= $24

Cost of 15 pens from Shop M
= 15 x 24
= $360

Price of the 6th pen from Shop N in percentage
= 100% - 30%
= 70%

Cost of 6th pen at Shop N
= 70% x 30
= ⁷⁰₁₀₀ x 30 
= $21

Cost of 1 set of 6 pens at Shop N
= 30 x 5 + 21
= $171

Number of sets of 6 for pens at Shop N
= 15 ÷ 6
= 2 r 3

Cost of 2 sets of 6 pens at Shop N
= 2 x 171
= $342

Cost of 3 pens at Shop N
= 3 x 21
= $63

Cost of 15 pens at Shop N
= 342 + 63
= $405

Amount cheaper if Penelope buys 15 pens from Shop M instead of Shop N
= 405 - 360
= $45

Answer(s): $45