The usual price of a pencil sold at Shop C and D is $10. During a sale, a discount of 20% is given to every pencil sold at Shop C. At Shop D, a discount of 40% is given to the 7th pencil for every 7 pencils sold. How much cheaper would it be if Diana buys 16 pencils from Shop C instead of Shop D?

	Shop C	Shop D
Price of 1 pencil	80%  $8	100% $10
Every 7th pencil		60% $6

Price of 1 pencil from Shop C in percentage
= 100% - 20%
= 80%

Price of 1 pencil from Shop C at 20% discount
= 80% x 10
= ⁸⁰₁₀₀ x 10 
= $8

Cost of 16 pencils from Shop C
= 16 x 8
= $128

Price of the 7th pencil from Shop D in percentage
= 100% - 40%
= 60%

Cost of 7th pencil at Shop D
= 60% x 10
= ⁶⁰₁₀₀ x 10 
= $6

Cost of 1 set of 7 pencils at Shop D
= 10 x 6 + 6
= $66

Number of sets of 7 for pencils at Shop D
= 16 ÷ 7
= 2 r 2

Cost of 2 sets of 7 pencils at Shop D
= 2 x 66
= $132

Cost of 2 pencils at Shop D
= 2 x 6
= $12

Cost of 16 pencils at Shop D
= 132 + 12
= $144

Amount cheaper if Diana buys 16 pencils from Shop C instead of Shop D
= 144 - 128
= $16

Answer(s): $16