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.

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.

Quote from: Redz on October 10, 2010, 03:59:18 PM

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.