The usual price of a pencil sold at Shop W and X is $40. During a sale, a discount of 25% is given to every pencil sold at Shop W. At Shop X, a discount of 50% is given to the 4th pencil for every 4 pencils sold. How much cheaper would it be if Shiyun buys 15 pencils from Shop W instead of Shop X?

	Shop W	Shop X
Price of 1 pencil	75%  $30	100% $40
Every 4th pencil		50% $20

Price of 1 pencil from Shop W in percentage
= 100% - 25%
= 75%

Price of 1 pencil from Shop W at 25% discount
= 75% x 40
= ⁷⁵₁₀₀ x 40 
= $30

Cost of 15 pencils from Shop W
= 15 x 30
= $450

Price of the 4th pencil from Shop X in percentage
= 100% - 50%
= 50%

Cost of 4th pencil at Shop X
= 50% x 40
= ⁵⁰₁₀₀ x 40 
= $20

Cost of 1 set of 4 pencils at Shop X
= 40 x 3 + 20
= $140

Number of sets of 4 for pencils at Shop X
= 15 ÷ 4
= 3 r 3

Cost of 3 sets of 4 pencils at Shop X
= 3 x 140
= $420

Cost of 3 pencils at Shop X
= 3 x 20
= $60

Cost of 15 pencils at Shop X
= 420 + 60
= $480

Amount cheaper if Shiyun buys 15 pencils from Shop W instead of Shop X
= 480 - 450
= $30

Answer(s): $30