Need some math help.

If items underwater look like they are 3' away but are really 4' away how much closer do they appear using a % to state it? Also please explain your answer.

Thanks
JT

Umm I am not so sure, but wouldnt it be 25% closer? 25 goes into 100 4 times, yadda yadda and so... okay, its hard to explain myself and I am quite confused now... Will edit this post after I do the work on paper...

Edit: Yes, its 25%. 3, is the fake so I labeled it as "F". 4 is the real so I labeled it "R". it is really 4' away, so lets call that 100 or 100%. it looks 3' away although it really isnt. so it is 25% closer and that would subtract 1/4 which is one out of the four, as in 3 out of 4. Or am I just stupid? I did get some good grades in math this year... and your hearing this from a 12 year old...

Did you post this just to stir up our minds and make us cuss and swear and hit our own heads trying to figure it out? Because if you did, it worked really well. /ubbthreads/images/graemlins/grin.gif

Well, lets start with establishing a common understanding so we can communicate.

Do you understand fractions?

Do you understand ratios?

Do you understand percentages?

Do you understand how to convert a fraction to a precentage?

Is this your homework?

Are you designing a bridge, possibly from Oakland to San Francisco?

Daniel

3/4 = 0.75 or 75%. So something is going to appear to be 25% further away than it really is (or 75% closer if that makes sense). Using this logic something underwater that is 100 feet away will look like it is 75 feet out. (100 feet * 75% = 75 feet away). *All of this is IMO.

-Mike

Both Mags's and xrunner's ideas are the best I can figure. /ubbthreads/images/graemlins/rolleyes.gif Seems very logical.

We happen to be learning about converting fractions to ratios, and vice versa in math class these days... Also, fractions to percents, and vice versa.

Well Mags, if you're there you shouldn't be to far away from dividing,multiplying, adding, and subtracting poly and trinomials. I got stuck on those for a while. They aren't very easy to learn. But, one thing that I *do* like about math is that once you are able to get the problems fully understood in your head, it seems quite easy to get them later on when you'll need that stuff for other problems. Unless you wait TOO long and forget them completely. /ubbthreads/images/graemlins/icon15.gif

Say Jtivat, why do you need this info anyway?

[ QUOTE ]
jtivat said:
If items underwater look like they are 3' away but are really 4' away how much closer do they appear using a % to state it? Also please explain your answer.

Thanks
JT

[/ QUOTE ]


Take the difference in the distances, which in the case is 1'. Divide that by the number you want to compare to. Then multiply by 100 to change to percent.

1'/3'=.33
.33*100=33%

So they are 33% farther than they appear.

Why did I use the 3' as a basis? Well, if you're looking at them you only know how far they appear to be and not how far they actually are.

To reverse this:
1'/4'=.25
.25*100=25%

So they appear 25% closer than they actually are.


************************************

Interesting math note:

Sometime working with small numbers (less than 10) is harder than large ones. Why? Numbers under 10 are more likely to fit special rules.

Example:
we know that 2+2=4
we also know that 2*2=4

I was doing a math problem one time and punched 3.5*1.4 into my calculator. It said the answer was 4.9

What's wrong with this? In my mind I could quickly tell than 3.5+1.4 was 4.9 so I figured I had hit "add" instead of "multiply". I checked again. Same answer.

3.5+1.4=4.9
and
3.5*1.4=4.9

Interesting!

For those interested in this....
x/(x-1) will yield the 2nd number in the series.
3.5/2.5=1.4
3.5*1.4=3.5+1.4

6/5=1.2
6*1.2=7.2
6+1.2=7.2

[ QUOTE ]
Xrunner said:
3/4 = 0.75 or 75%. So something is going to appear to be 25% further away than it really is (or 75% closer if that makes sense). Using this logic something underwater that is 100 feet away will look like it is 75 feet out. (100 feet * 75% = 75 feet away). *All of this is IMO.

-Mike

[/ QUOTE ]

Not quite...

It will be 33 1/3% farther (take 1/3 of 3 and add it to 3, or 4), or 25% closer. it all depends on what you numerator is in the ratio you're setting up.

-F

I am working on my Dive Master certification and disagreed with one of the questions. They claim things look 25% larger underwater and that an item 4' away looks like it is 3' away. Then there was a question on the test that asked what % closer things look underwater. I though the question was BS as it is a DM test not a math test and they never told us it was 33% they only stated the 3' @ 4' statement. I was pissed as it was the only question I missed on the test, and even the instructor could not explain the math to me.

As you can see, there are multiple interpretations depending on exactly how it's phrased. I always have to think about it too. You'd think the only rule you'd really need is "things are closer than they appear".

What percentage do you need to get correct to get your Dive Master cert? Is it like everything else, 70%?


Daniel

There are eight test and you need to get 75% on them all. I think I got 95% on the first one but as I got the % question wrong who knows. /ubbthreads/images/graemlins/smile.gif

Just out of curiosity, what agency are you doing it with? PADI?

[ QUOTE ]
scuba said:
Just out of curiosity, what agency are you doing it with? PADI?

[/ QUOTE ]

Yes, but not b/c I like PADI better than the others. I think it is all in the instructor and I really like my instructor, as he is very patient and will spend as much time with a student as they need to get it right.

jtivat,
According to the PADI open water handbook.
Refraction results in magnification of about 25%.
This gives the 4' & 3' example you referred to.

Depending on the exact wording of the question the correct answer could be different.

How much CLOSER does an object appear underwater?
Answer 25%

How much FARTHER away is an object than it appears underwater? Answer 33%

They are the same situation, 4' away & appear to be 3' away.
The answer changes based on the perspective of the question.

Now examine the question you missed & see if it's correct, or did the test writer get the two questions & answers commingled?

Theepdinker

So was I correct about 25% closer? All this math is making me want to just lie down for a while.....

[ QUOTE ]
Mags said:
So was I correct about 25% closer? All this math is making me want to just lie down for a while.....

[/ QUOTE ]

Nope there answer was 33%.

So, 33% closer and 25% farther away. That's kinda confusing but I think I get it now.

Oh so I got it mixed up... now I understand.... lying down time is shortened to 3 hours...