The regular selling price of an article at Acme Retailers is equal to the cost price of the article plus a markup of m%. When the owner of Acme Retailers agrees to sell 16 articles for the regular selling price of 12 articles plus a 20% discount, he still makes a profit of 20%. What is the value of m?

a) 100%
b) 80%
c) 75%
d) 66+2/3%
e) 55%

I've changed the wording to clarify what I believe is the intent of the problem.

We can plug in the answers, which represent the markup.
Given that both percentages in the problem are multiples of 10, the correct answer choice also is likely a multiple of 10.
Of the two multiples of 10 in the answer choices -- 100 and 80 -- the easier one to evaluate is 100.

Answer choice A: 100% markup
Let the cost per article = 10.
Cost of 16 articles = 16*10 = 160.
Regular selling price of 12 articles with a 100% mark-up = 12*10 + (100/100)(12*10) = 240.
Selling price with a 20% discount = 240 - .2(240) = 240-48 = 192.
Profit = 192-160 = 32.
Profit/cost = 32/160 = 1/5 = 20%,
Success!

The correct answer is A.

Just wondering,is this a generic rule

Given that both percentages in the problem are multiples of 10, the correct answer choice also is likely a multiple of 10.
Of the two multiples of 10 in the answer choices -- 100 and 80 -- the easier one to evaluate is 100.

Algebraic approach:

Let CP = x => SP = x{1+m/100}

Selling price of 12 articles = 12 * x{1+m/100}

Since 20% discount was given => articles were sold at 80% of SP

New selling price of 12 articles = 12 * 0.8 * x{1+m/100}

To determine profit, we have to subtract SP from CP.

Since, 16 items were sold we have to subtract CP of 16 items from SP of 12 items.

=> Profit = SP - CP = 12 * 0.8 * x{1+m/100} - 16x

Given profit is 20% => 0.2 * 16x

0.2 * 16x = 12 * 0.8 * x{1+m/100} - 16x

On solving we get m = 100

Thus A.