The usual price of a notebook sold at Shop C and D is $36. During a sale, a discount of 25% is given to every notebook sold at Shop C. At Shop D, a discount of 50% is given to the 5th notebook for every 5 notebooks sold. How much cheaper would it be if Yoko buys 13 notebooks from Shop C instead of Shop D?

	Shop C	Shop D
Price of 1 notebook	75%  $27	100% $36
Every 5th notebook		50% $18

Price of 1 notebook from Shop C in percentage
= 100% - 25%
= 75%

Price of 1 notebook from Shop C at 25% discount
= 75% x 36
= ⁷⁵₁₀₀ x 36 
= $27

Cost of 13 notebooks from Shop C
= 13 x 27
= $351

Price of the 5th notebook from Shop D in percentage
= 100% - 50%
= 50%

Cost of 5th notebook at Shop D
= 50% x 36
= ⁵⁰₁₀₀ x 36 
= $18

Cost of 1 set of 5 notebooks at Shop D
= 36 x 4 + 18
= $162

Number of sets of 5 for notebooks at Shop D
= 13 ÷ 5
= 2 r 3

Cost of 2 sets of 5 notebooks at Shop D
= 2 x 162
= $324

Cost of 3 notebooks at Shop D
= 3 x 18
= $54

Cost of 13 notebooks at Shop D
= 324 + 54
= $378

Amount cheaper if Yoko buys 13 notebooks from Shop C instead of Shop D
= 378 - 351
= $27

Answer(s): $27