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Sentence Correction: if/whether (or not)

When calculating the annual tax deducted at the end of the year, it is sometimes difficult to predict whether an income will surpass a certain bracket.

This answer choice is stylistically flawed. Using both annual and year is redundant.

What helps us identify this question as a Redundancy question as well as identify this mistake is the following Stop Sign:  

annual + year

Although this answer choice corrects the original Redundancy error by not using both annually and a year, in the GMAT, the word if is used in Conditionals only; if one thing, ...then something else. Since in this question there is no condition, if should  be replaced by whether.

Whether is used as a choice between two possibilities dealing with the same subject or as a yes/no question.

Example: I do not know whether this will work. (yes or no)

He was not sure whether he wanted banana or chocolate cake. (a choice between 2 possibilities)

Although this answer choice corrects the original Redundancy error by not using both annually and a year, in the GMAT, the addition of the phrase or not at any point after the word whether is considered redundant.

Although this answer choice corrects the original Redundancy error by not using both annually and a year, in the GMAT, the addition of the phrase or not at any point after the word whether is considered redundant.

Great work!

Using both annual and year is redundant. This answer choice corrects the error by removing annual.

When calculating the annual tax deducted at the end of the year, it is sometimes difficult to predict whether an income will surpass a certain bracket
When calculating the tax deducted at the end of the year, it is sometimes difficult to predict if an income will surpass a certain bracket
It is sometimes difficult to predict whether or not an income will surpass a certain bracket when calculating the tax deducted annually
When calculating the tax deducted at the year's end, it is sometimes difficult to predict whether an income will surpass a certain bracket
When calculating the tax deducted at the end of the year, it is sometimes difficult to predict whether an income will surpass a certain bracket or not