Just when I thought I was getting better at math...

[shadowkat]

This

Post by Power 95.3.

made me scratch my head.


7 + 7 divided by 7 +7 x7 -7.

The correct answer is apparently 50.

IF you do it this way: 7+7 =14 divided by 7 =2
2 +7 = 9, 9* 7 =63, 63-7 = 56.

If you do it by calculator = 56

If you do it by Excel = 50

Apparently it is supposed to be 7x7= 49 /7 = 1, then 1+7+7 -7 = 8.
Which still makes no sense. So, it must be = 7x7/7 =1, then 7+7+7 =21 +1
Still makes no sense. I remain puzzled.

This is why I hate math.

Comments

[cactuswatcher.livejournal.com]

My algebraic calculator a TI-36 (which I rarely use) gives 50. Very disturbing that you have one that gives the wrong answer!

[cactuswatcher.livejournal.com]

Now that I think about it there was a discussion in Scientific American magazine, I think, about 20 years ago about some algebraic calculators which would give bad answers like this on chained operations. I guess that problem has not been eliminated completely.

[shadowkat67.livejournal.com]

Just tried it on my calculator - it won't wait for the next, or for me to enter an = sign. I plug in 7+7 and I get 14, before I even can press / by 7.
In order for it to work, I have to do this:

7/7 =1
7x7=49
1+7+49-7 =50

And I have to press "clear" really hard between calculations. But it's also a dirt cheap basic calculator provided by the government. Not a fancy one.

[flake-sake.livejournal.com]

The old ones from about 15 years ago all do that. If you know it, you can always insert brackets to make up for it and it is not much of a problem.

The true algebraic ones usually have the rule implemented.

ETA: And I think even the newer normal ones for school do. I checked with a current TI36 (but not pro) from a tutoring kid and it does it perfectly fine.

[shadowkat67.livejournal.com]

The new cheap one's don't - because the one I recently got through work comes up with 56, unless I separate out the calculations. Granted, I rarely if ever have to do complicated algebraic equations. And if I do - it's in excel.

I rarely use the calculator for them.

[flake-sake.livejournal.com]

Yep, I usually use matlab for algebra. My calculator can't do it. You can enter a string of calculations but you can't calculate with letters.

I kind of would like to have one of those cool algebraic graphical calculators, but they are not exactly cheap considering that every cell phone has more power than they do these days.

[shadowkat67.livejournal.com]

Yeah, as my cousin stated - this problem was a really good test of calculators. Excel is your friend. Except Excel automatically rounds up, which drives my co-workers crazy.

[flake-sake.livejournal.com]

Excel is good for most stuff, but in some labs you are not allowed to use it because weirdly enough it's results are not always reproducable. If you transfer files between computers, the data can change depending on settings.

For some uses in the pharmaceutical industry it's explicitly forbidden to keep the records in excel for this reason.

You can make it round correctly, but its a bit of procedure for such a simple thing.

[shadowkat67.livejournal.com]

I've discovered that as well. In my industry it's fine - procurement of railroad environmental engineering, construction and consulting services. But I've had to double check it on the rounding and had to turn off the rounding feature. Often the consultants that I negotiate with - don't do this, which means their figures are always slightly off.

That's why I think it's important to understand the calculations that EXCEL is completing so you can do it separately if required. Between you, catcuswatcher, and my cousin, I finally understood why 50 was the correct answer. Thanks for that by the way.