We are given that the revenue from the sales of jazz and blues in 2000 accounted for 0.06 x (840 million) = 50.4 million dollars in revenue. We need to determine the revenue from the sales of jazz and blues in 1998.

Statement One Alone:

Jazz and blues recordings accounted for 5 percent of the revenue from the sales of recordings in Country Y in 1998.

Since we do not have any information about the total revenue in 1998, statement one alone is not sufficient to answer the question. We can eliminate answer choices A and D.

Statement Two Alone:

The revenue from the sales of jazz and blues recordings in Country Y increased by 40 percent from 1998 to 2000.

Using the information given and statement two, we can determine the revenue from sales of jazz and blues in 1998. If we let the revenue from sales of jazz and blues in 1998 = R, we can create the following equation:

1.4R = 50.4 million

R = (50.4 million)/1.4

R = 36 million

Statement two alone is sufficient to answer the question.

Answer: B
_________________

Jazz and blues recordings accounted for 6 percent of the $840 million revenue from the sales of recordings in Country Y in 2000. What was the revenue from the sales of jazz and blues recordings in Country Y in 1998?

(1) Jazz and blues recordings accounted for 5 percent of the revenue from the sales of recordings in Country Y in 1998:

We dont know the revenue from the sale of recordings in 1998 and cant calculate revenue from jazz and blues recordings. INSUFFICIENT

(2) The revenue from the sales of jazz and blues recordings in Country Y increased by 40 percent from 1998 to 2000

Revenue in 2000 = $50.4 million
Let revenue in 1998= x

x + 0.4 * x =$50.4
x=$36 million

SUFFICIENT

Ans:B
_________________

Appreciate any KUDOS given !

Focusing on the relationship of variables helps to see sufficiency in percentage DS questions. Often they can confuse you to think you have info or more than often you don't have enough information.

Stem gives us 2000 Revenue = .06 * 840 Million

We require revenue in 1998 by jazz & blues
Statement 1 : Gives us Revenue Percentage of 1998 with no amount of that year. Insufficient
Statement 2 : Gives us increase in Revenue from 1998 to 2000. Which translates to => x + .4x = .06 * 840 Million. Sufficient

From statement 2 alone we can get x ( revenue of 1998)

Hence Answer is B.

Writing down what is required clearly on paper, will help a lot in terms of recognizing what relationship you need for sufficiency.
Further breaking that down ahead increases accuracy.