Discount

let x be the selling price of a dozen of pens
x - x(1-45%) = 27 ...求左1 打既售價先 ,題目話折扣既45% 就平27元 ,27元係售價減折完平左既數
x - 0.55x = 27
x (1- 0.55 ) = 27
0.45x = 27
x = $60 ....一打鉛筆售60元 ,但不是答案 ,仲要除左12先
the selling price of each pen is $5 (= $60/12)

設Y為一打筆

Y * 45% = $27
Y = $27 / 45%
Y = $60

一打筆為$60
一支筆為$60 / 12 = $5

let $y be the selling price of each pen.
12y*45%=27
12y=30
y=2.5
the selling price of each pen is $2.5