Math: Trying to Help My Daughter w/ PSATs

[Neurotic Guy]

I am OK up to about 7th grade math.  After that, I am pretty much done.  My younger daughter is preparing for PSATs next week and we've been going through practice tests.

Math isn't her strong suit either and we've struggled with a lot of the questions - giving up on the more advanced math. But, here is one we both thought we knew -- one of the easy ones -- but we got it wrong and can't figure out why:

If p= -5 and q= -4, then p(p-q)= ?
The answer we both came up with is 5.  The correct answer is 45.
Can't figure out why it doesn't play out like this:
-5(-5+4)=5.  It must be -5(-5-4)=45, but I don't get why it's  -4 rather than +4.

Please explain.

[BballTim]

I am OK up to about 7th grade math.  After that, I am pretty much done.  My younger daughter is preparing for PSATs next week and we've been going through practice tests.

Math isn't her strong suit either and we've struggled with a lot of the questions - giving up on the more advanced math. But, here is one we both thought we knew -- one of the easy ones -- but we got it wrong and can't figure out why:

If p= -5 and q= -4, then p(p-q)= ?
The answer we both came up with is 5.  The correct answer is 45.
Can't figure out why it doesn't play out like this:
-5(-5+4)=5.  It must be -5(-5-4)=45, but I don't get why it's  -4 rather than +4.

Please explain.

  Looks like a typo.

[Redz]

negative 5 minus negative 4 = -9

a negative x a negative =  positive

so -5 x -9 = 45

the "double negative" doesn't apply to subtraction like it does multiplication or division

[Redz]

think about like this.  if p =-5, what is 4 less than P? less with negatives gets further away from zero.



try drawing a number line with both sides of zero and visualize it that way

[wdleehi]

From a high school Physics/Math teacher, there is something wrong with how the problem is written.




Either it is suppose to be a positive 4 or an addition sign.  What you wrote there is equal to 5.

[Roy H.]

I think you might be mistaken, Redz.  I think it's a typo, as "q" needs to be "4", rather than "-4", for this to make any sense.

http://www.mathleague.com/help/posandneg/posandneg.htm

[Neurotic Guy]

The issue isn't with adding/subtracting negative numbers. 

The issue is, if q = -4 why isn't the resulting equation:
-5(-5 - -4) leading to -5(-5+4) to -5(-1)=5.  If we were correct, the result is 5 (not 45 as the answer key states).  It sounds like most here think that the book is incorrect.  It was driving us crazy so it would be nice to know that it's a typo rather than some strange math thing. 

[Neurotic Guy]

From a high school Physics/Math teacher, there is something wrong with how the problem is written.




Either it is suppose to be a positive 4 or an addition sign.  What you wrote there is equal to 5.

Thanks! 

[Tai]

Yeah, math wiz here too. I see 5 has the answer all the way.

[Tai]

And just so you know, I'm in a college algebra class this semester (yes, college yay!) and the answer booklet has SEVERAL typos. I get the right answer, even if the answer booklet doesn't. So yeah, I wouldn't be surprised if the SAT did the same.

[Neurotic Guy]

And just so you know, I'm in a college algebra class this semester (yes, college yay!) and the answer booklet has SEVERAL typos. I get the right answer, even if the answer booklet doesn't. So yeah, I wouldn't be surprised if the SAT did the same.

Thanks -- I have too much faith in the printed word and too little faith in my math ability to question the text.  Good to know our math was correct, but makes me wonder if my daughter is studying with an unreliable source. 

[Redz]

hmmm

good thing we don't cover that in 5th grade 

[pearljammer10]

negative 5 minus negative 4 = -9

a negative x a negative =  positive

so -5 x -9 = 45

the "double negative" doesn't apply to subtraction like it does multiplication or division

Sorry, this my friend is not correct. negative 5 minus negative four with always equal negative one. There is a typo in the exam, unfortunately this happens a decent amount of the time.

[Redz]

negative 5 minus negative 4 = -9

a negative x a negative =  positive

so -5 x -9 = 45

the "double negative" doesn't apply to subtraction like it does multiplication or division

Sorry, this my friend is not correct. negative 5 minus negative four with always equal negative one. There is a typo in the exam, unfortunately this happens a decent amount of the time.

yup

I stand corrected.  Boo me.

If it's any excuse I had a headache at the time.  

still do, come to think of it

[Neurotic Guy]

negative 5 minus negative 4 = -9

a negative x a negative =  positive

so -5 x -9 = 45

the "double negative" doesn't apply to subtraction like it does multiplication or division

Sorry, this my friend is not correct. negative 5 minus negative four with always equal negative one. There is a typo in the exam, unfortunately this happens a decent amount of the time.

yup

I stand corrected.  Boo me.

If it's any excuse I had a headache at the time.  

still do, come to think of it

Hey -- no shame in making a math error.  Math makes my head hurt.