The usual price of a notebook sold at Shop P and Q is $10. During a sale, a discount of 20% is given to every notebook sold at Shop P. At Shop Q, a discount of 30% is given to the 4th notebook for every 4 notebooks sold. How much cheaper would it be if Gwen buys 10 notebooks from Shop P instead of Shop Q?

	Shop P	Shop Q
Price of 1 notebook	80%  $8	100% $10
Every 4th notebook		70% $7

Price of 1 notebook from Shop P in percentage
= 100% - 20%
= 80%

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

Cost of 10 notebooks from Shop P
= 10 x 8
= $80

Price of the 4th notebook from Shop Q in percentage
= 100% - 30%
= 70%

Cost of 4th notebook at Shop Q
= 70% x 10
= ⁷⁰₁₀₀ x 10 
= $7

Cost of 1 set of 4 notebooks at Shop Q
= 10 x 3 + 7
= $37

Number of sets of 4 for notebooks at Shop Q
= 10 ÷ 4
= 2 r 2

Cost of 2 sets of 4 notebooks at Shop Q
= 2 x 37
= $74

Cost of 2 notebooks at Shop Q
= 2 x 7
= $14

Cost of 10 notebooks at Shop Q
= 74 + 14
= $88

Amount cheaper if Gwen buys 10 notebooks from Shop P instead of Shop Q
= 88 - 80
= $8

Answer(s): $8