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

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

Price of 1 file from Shop D in percentage
= 100% - 25%
= 75%

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

Cost of 13 files from Shop D
= 13 x 27
= $351

Price of the 5th file from Shop E in percentage
= 100% - 50%
= 50%

Cost of 5th file at Shop E
= 50% x 36
= ⁵⁰₁₀₀ x 36 
= $18

Cost of 1 set of 5 files at Shop E
= 36 x 4 + 18
= $162

Number of sets of 5 for files at Shop E
= 13 ÷ 5
= 2 r 3

Cost of 2 sets of 5 files at Shop E
= 2 x 162
= $324

Cost of 3 files at Shop E
= 3 x 18
= $54

Cost of 13 files at Shop E
= 324 + 54
= $378

Amount cheaper if Tammy buys 13 files from Shop D instead of Shop E
= 378 - 351
= $27

Answer(s): $27