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gmatophobia

XimeSol
Lea estimated each of the charges on her telephone bill by rounding the charge to the nearest $1. Was the sum of Lea's estimated charges within $8 of the sum of the actual charges on her telephone bill?

(1) Each of the actual charges on Lea's telephone bill was less than $7.75

(2) There were 14 charges on Lea's telephone bill.

A)Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B)Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C)BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D)EACH statement ALONE is sufficient.
E)Statements (1) and (2) TOGETHER are NOT sufficient.

Statement 1

(1) Each of the actual charges on Lea's telephone bill was less than $7.75

While we know the actual charge on each of the bills was less than $7.75, we do not know the number of charges that we are looking at.

For example, Lea rounded two charges. The first charge was for $4.45, and the second charge was for $5.90. She rounds the first charge to $4.00, and she rounds the second charge to $6.00. In this case, the actual sum is $(4.45 + 5.90 =) 10.35, while the rounded sum is $10.00

Was the sum of Lea's estimated charges within $8 of the sum of the actual charges on her telephone bill → The answer is Yes.

On the contrary, let's consider that Lea rounded a million charges. Each charge was for $5.90, therefore she rounded each charge to $6.00.

In this case, the actual charge is $5.9M, while the rounded charge is $6M.

Was the sum of Lea's estimated charges within $8 of the sum of the actual charges on her telephone bill → The answer is No.

As we are getting contradicting answers to the question asked, this statement is not sufficient. We can eliminate A and D.

Statement 2

(2) There were 14 charges on Lea's telephone bill.

If there were 14 charges on Lea's telephone bill the maximum difference between the actual sum and the rounded sum can be $7.

To understand this better consider this statement, we need to consider the scenarios when the difference between the actual and the rounded charge is expected to be maximum. The maximum difference between the actual sum and the rounded sum occurs when the actual sum is rounded down from \(x.49\) to \(x-1\) or when the sum is rounded up from \(x.50\) to \(x+1\). In both scenarios, the maximum difference between the actual sum and the rounded sum is $0.50.

If Lea rounded 14 charges, the maximum between the actual sum and the rounded sum can be 14*0.50 = $7

Was the sum of Lea's estimated charges within $8 of the sum of the actual charges on her telephone bill → The answer is Yes.

This statement therefore is sufficient.

Option B
­The explanation and the foot note of power of tiny gains resonate
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if she rounds to the nearest dollar, then that means the largest margin of error she could have is 50 cents. This is because you either round up or down after this. Therefore with 14 charges each with a max of 50 cent margin of error, that means the max cumulative error would be 0.50 * 14 = 7. 7 is within an $8 margin of error.
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KarishmaB Bunuel @chetan4u Please help with a better explanation of this question
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Very neat little question.

Per the prompt, we are basically trying to figure out if the sum of each individual charge estimate is within $8 of the sum of the actual individual charges.

Each of the actual charges on Lea's telephone bill was less than $7.75.
This is insufficient simply because it tells us nothing about the number of charges.
If there was one charge that was $7.74, then the difference between what was estimated and the actual would be $0.26. In other words, you would have overestimated the actual but still be within $8. This would be your "yes."

The "no" scenario would be if there were, say, 40 billion charges at $7.50 each. Obviously, the difference in that case would be enormous and clearly more than $8.

There were 14 charges.
Let's say the actual charges were each $0.01, so the total is $0.14. Rounding each charge down to $0 would give an estimated total of $0, and $0.14 - $0 is clearly less than $8.

Alternatively, suppose there were 14 charges at $1.50 each, totaling $21. Each would be rounded up to $2, making the estimate $28. The difference is $7, which is less than $8.

So we have our proof: if the maximum difference per charge is $0.50, then for 14 charges the total maximum difference is $7. Since $7 < $8, statement (2) is sufficient.
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XimeSol
Lea estimated each of the charges on her telephone bill by rounding the charge to the nearest $1. Was the sum of Lea's estimated charges within $8 of the sum of the actual charges on her telephone bill?

(1) Each of the actual charges on Lea's telephone bill was less than $7.75

(2) There were 14 charges on Lea's telephone bill.

A)Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B)Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C)BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D)EACH statement ALONE is sufficient.
E)Statements (1) and (2) TOGETHER are NOT sufficient.

When we round to the nearest 1, we round down (to x) if the number is less than x.5 and we round up (to x+1) if the number is greater than or equal to x.5. Hence maximum error in one reading can be 0.5.

Question: Was the total rounding error <= $8

(1) Each of the actual charges on Lea's telephone bill was less than $7.75

This doesn't tell us anything. We are evaluating rounding of numbers. A bill less than 7.75 could be anything - say 5.4, or 6.5 etc. Do we round up or down and by how much, no idea at all.

(2) There were 14 charges on Lea's telephone bill.


We said above that the maximum error in one reading can be 0.5. So maximum error in all the 14 readings combined can be 0.5 * 14 = 7
(e.g. if each reading is 5.5 and is rounded up to 6)

Hence the total error is certainly less than $8.

Answer (B)
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