Question

Ralph buys a desk that regularly sells for $250 and a desk chair that regularly sells for $145. both items are on sale at 15% off, and a sales tax of x% is applied to both. Which expression does not represent the amount Ralph pays? Show your work.
A. (395)(0.0085x)+395(0.85)

B. (250+145)(1-0.15)(100+x/100)

C.(0.85+x/100)(250+145)

D.335.75+3.3575x

Answer 1

C) (0.85 + x/100)(250+145) does not give the correct answer.

Explanation
A) works; adding the two costs together is 250+145=395.  We multiply this by 0.85 because 100%-15%=85%=0.85.  We also have x% tax, which is represented by x/100, added to 100% of the value, or 1.00.  Altogether this gives us 
395(0.85)(1+x/100) = 395(0.85 + (0.85x/100)) = 395(0.85) + 395(0.85x/100)
= 395(0.85) + 395(0.0085x)
B) works; we have 250+145 for the original price; we have 85% = 0.85 of the value; we also have 100% + x%, which is (100+x)/100.
C) does not work; (0.85+x/100)(395) does not take into consideration that you are finding the tax after taking the 85%.  This will simplify out to
0.85*395 + (x/100)(395) = 335.75 + 395x/100 = 335.75 + 3.95x, which is too much.
D) works; simplifying the expression from A, we have 395(0.85) + 395(0.0085x) = 335.75 + 3.3575x.