pushpitkc wrote:

The minimum price per book is 1$.

Since each of the friends need to buy 1 book, and the total cost of the books is 25$

the costliest book that could be bought is 21$(since the other books are bought at 1$ each)

Hence, Option D is the correct answer!

P.S It is very easy to mistake "No two friends paid the same amount for a book"

to mean "no two friends bought a book for the same price"

pushpitkc , I don't follow your logic in the P.S.

Take the essence of each of your sentences and write each sentence in the corresponding singular affirmative case (where each of four friends A, B, C, and D parts with one dollar for one book):

"No two friends paid the same amount for a book."

A paid the amount of $1 for a book.

B paid the amount of $1 for a book.

C paid the amount of $1 for a book.

D paid the amount of $1 for a book.

It appears to me that A and B paid the same amount, as did A and C, A and D, B and C, B and D, and C and D. There are six cases that seem to defy the rule that "No two friends paid the same amount for a book."

Do you mean that the sentence suggests that three and/or four friends can pay the same amount, but not "just" two?

The other sentence: "No two friends bought a book for the same price."

A bought the book for the price of $1.00

B bought the book for the price of $1.00

C bought the book for the price of $1.00

D bought the book for the price of $1.00

I see neither the logical difference between the two sets of sentences nor the difference between their corresponding antecedent sentences.

If you are distinguishing between "paid [an] amount for," and "bought for [a] price of," what is the distinction?

What am I missing here?

_________________

In the depths of winter, I finally learned

that within me there lay an invincible summer.

-- Albert Camus, "Return to Tipasa"