QR probability rules

[hokie4life]

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This is gonna sounds really silly but I keep messing this up! Is there a generalization that can be made as to when we multiply and when we add probabilities? I thought itw as

-when we say AND we MULTIPLY
-when we say OR we ADD

But sometimes it doesn't work out like that e
ex:

Jean’s net salary is $175 per week. If she spends 1/3
on food, 2/5 on rent and 1/6 on other expenses, how
many weeks will it take her to save at least $200?

to get the answer they added 1/3 on food, 2/5 on rent and 1/6 on other expenses but trying to use those generalized rules I wanted to multiply cuz it said AND.

So is this question just off or can we not generalize? Also even in bio probability it says that if the outcome of the second event is dependent on the first you multiply, so again shoudln't we mutliply?​

 

[ArmyOrtho1Day]

Well I've never heard of the rules you're referring to, but if you just think through it logically and in simple terms, it's pretty simple. I'm sure you've already gotten the answer, but its 12 weeks (11.4, but you don't get payed 3 days through the last week). I'm inclined to say though that you can't generalize it, mainly because words can always be switched around to mean other things. It's just dependent on how the author wants to use them.

 

[whoaaitzkyle]

This is gonna sounds really silly but I keep messing this up! Is there a generalization that can be made as to when we multiply and when we add probabilities? I thought itw as

-when we say AND we MULTIPLY
-when we say OR we ADD

But sometimes it doesn't work out like that e
ex:

Jean’s net salary is $175 per week. If she spends 1/3
on food, 2/5 on rent and 1/6 on other expenses, how
many weeks will it take her to save at least $200?

to get the answer they added 1/3 on food, 2/5 on rent and 1/6 on other expenses but trying to use those generalized rules I wanted to multiply cuz it said AND.

So is this question just off or can we not generalize? Also even in bio probability it says that if the outcome of the second event is dependent on the first you multiply, so again shoudln't we mutliply?​

Don't generalize, it will hurt you. To avoid that method what you can do is take each individual number and add them together. If she makes $175 a week and she spends 1/3 of that on food then you know 1/3(175). Get that number and write it down. Then do it to the next (2/5)175. Add the three numbers you get and subtract them from $175 and you'll see how much she profits per week and then divide that by $200.

This sounds complicated and it is a lot more work than you need to but the way I figure is it avoids mistakes. You know you're doing it correct. If you are finding it difficult to interpret the problem I'd spend that extra 45 seconds to do the math so you know it's right.

The easiest way is to just know that they are all a portion of her salary so add them all up and take the fraction of that but if you are the least bit confused at how to approach the problem when you see it I would take the longer route. Both ways you get the same answer.

 

[Flipper405]

I agree with the other posters; you definitely don't want to rely on generalizations. First of all, this isn't even a probability question, so it wouldn't apply anyway. But even if it were, you should have much more success if you interpret the problem and figure it out that way. I did it in a similar way as the poster above me, and it prevents you from making other mistakes.

 

[Streetwolf]

Like ^poster said, this is not a probability question. It's a fraction question.

A probability question would sound like:

What are the odds that events A and B will occur? <-- you would multiply.

 

[hokie4life]

thanks guys this advice helps! i will try understanding more and generalizing less. I guess with everyones scary QR scores it's worth it to spend the extra time and get it right and not take shortcuts cuz it can easily mess you up if you don't even understand the problem

goodness when did highschool math get so hard 😉

 

[dentz]

I just added all the fractions and multiplied it by 175 to see how much she spent. Then I subtracted it from 175 to get the profit and divided that by 200. Is the answer around 12 weeks or 11.42 or sumthing in those lines???

 

[whoaaitzkyle]

I just added all the fractions and multiplied it by 175 to see how much she spent. Then I subtracted it from 175 to get the profit and divided that by 200. Is the answer around 12 weeks or 11.42 or sumthing in those lines???

You did the right method...I'm too lazy to do the math but we were trying to come up with alternative methods to solve the equation without adding the fractions up....but you did the problem right 👍

 

[dentz]

You did the right method...I'm too lazy to do the math but we were trying to come up with alternative methods to solve the equation without adding the fractions up....but you did the problem right 👍

🙂 Thanks!!