Answer
c. buying it at a 10 percent discount without sales tax.
Step-by-step explanation
We are going to compare the 3 choices and then determine the most cost effective.
a. using a paid membership card to buy it at a 10 percent discount.
10% of $100 = 10/100×100 = $10
cost = $100 - $10 = $90
On top of this $90 there is the charges of the membership card.
b. buying it online at a 10 percent discount with a $5 shipping charge.
100%-10% = 90%
90% of 100 = $90
Cost: $90 + $5 = $95
c. buying it at a 10 percent discount without sales tax.
100% - 10% = 90%
90/100 × 100 = $90. This is the must cost effective method since there is no other cost involved.