Data Sufficiency

bpolley00 wrote:In The decimal 3.1de, d and e each represent a digit. If X represents the decimal 3.1de rounded to the nearest tenth, what is the value of x?
1) D=4
2) E=8

Target question: What is the value of x?

Given: X represents the decimal 3.1de rounded to the nearest tenth

Statement 1: D = 4
So, the decimal could by 3.140, 3.141, 3.142, 3.143, 3.144, 3.145, 3.146, etc
To round any of these decimals to the nearest tenth, we need only check the hundredths digit.
Since the hundredths digit is 4, ALL of these decimals round to be 3.1
In other words, X must equal 3.1
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: E=8
There are several different decimals that meet this condition. Here are two:
Case a: the numbers decimal is 3.128, in which case X = 3.1
Case b: the numbers decimal is 3.168, in which case X = 3.2
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer = A

Cheers,
Brent

bpolley00 wrote: 3.14E, Not sufficient. How could this be sufficient, you don't know what E is. In any sort of Mathematics taught from K- college 3.14 could go either way depending on E aka whether or not the hundredths digit is below or above 5. It could be 3.149, which rounds to 3.15 which rounds to 3.2 or it could be 3.141 which rounds down 3.14 which rounds to 3.1

bpolley, in your example, you are rounding two different numbers.

Yes, 3.149, rounded to the nearest hundredth, is 3.15
And yes, 3.15, rounded to the nearest tenth, is 3.2

But notice that in your second example, you are rounding 3.15 (not 3.149) to the nearest tenth.

Let's examine 3.149, rounded to the nearest tenth
To do this, we must only examine the hundredths digit only. Since the hundredths digit is 4, we must round DOWN to 3.1

Cheers,
Brent

bpolley00 wrote:Brent-

But For statement 1) 3.146, in standard mathematics you would round 3.146 to 3.15 which then rounds up, right? You don't just ignore the hundredths digit completely, ever. Unless of course I am miss informed. But, I mean let's say you are doing your physics homework and your answer comes down to 3.146 and the professor round to the nearest tenth, you would round it as 3.15 and then round to 3.2 right?

I guess, I get what you are saying, it just to me is completely unconventional. Thanks for the help sir.


-BP

Rounding doesn't work that way.
When we round 3.146 to the nearest tenth, we're really asking "Is 3.146 closer to 3.1 or 3.2?"
3.146 is closer to 3.1 than it is to 3.2, so we round to 3.1

The only anomaly occurs when we want to round 3.15 to the nearest tenth. Here, 3.15 is just as close to 3.1 as it is to 3.2
In these situations, we have a rule that says we round up.

So, when rounding a number to the nearest tenth, we must check the hundredths digit only.

Example:
What is 4.44444444444444444444444444444444444444444445 rounded to the nearest tenth?
Do we need to start at the very end of this decimal and start rounding up?
No. This number is closer to 4.4 than it is to 4.5. So, we round DOWN to 4.4


Cheers,
Brent

Brent-
that's a pretty sweet explanation of rounding; i've actually bookmarked it, in case other students have the same question later.

on the other hand-

Brent@GMATPrepNow wrote:Example:
What is 4.44444444444444444444444444444444444444444445 rounded to the nearest tenth?
Do we need to start at the very end of this decimal and start rounding up?
No. This number is closer to 1.1 than it is to 1.2, so this number, rounded to the nearest tenth, is 3.1

4.4444...45 --> 1.1-1.2 --> 3.1
Now I'm wondering what show you were watching while you wrote that paragraph; it must have been a good one. |: