Not getting the right answer

[bobisaka]

For the question:
9 (2p - 5) = 72

My solution is:
18p -45 = 72

18p -45 +45 = 72 + 45

18p = 117

18p/18 = 117/18

p = 6.5


The right answer in the textbook however, is:
p = 13/2

What am I doing wrong?

 

[bobisaka]

I think I got it.. 6.5 x 2 = 13 = 13/2 = 117/9

When i get to a decimal answer, should i always trying make it into a fraction for the answer?

 

[Romsek]

You should forget about using decimals unless they are specifically called for.

[MATH]9(2p-5) = 72\\ 2p-5 = 8\\ 2p=13\\ p=\dfrac{13}{2}[/MATH]

 

[bobisaka]

Was i incorrect in how i did my solution?

I understand that you did it an alternative method by dividing the constants on either side by 9.
How should i know which method to use? does it matter?

When solving equations should i be using different methods to solving it? (as long as it follows the rules)

 

[Harry_the_cat]

Was i incorrect in how i did my solution?

I understand that you did it an alternative method by dividing the constants on either side by 9.
How should i know which method to use? does it matter?

When solving equations should i be using different methods to solving it? (as long as it follows the rules)

Your method is perfectly ok. It really doesn't matter, just make sure you expand correctly (as you did).
Your answer is correct since 6.5 and 13/2 are the same.

 

[bobisaka]

Okay, thanks for the clarification.

 

[Steven G]

I think I got it.. 6.5 x 2 = 13 = 13/2 = 117/9

When i get to a decimal answer, should i always trying make it into a fraction for the answer?

13 = 13/2 is NOT true and 13/2 = 117/9 is NOT true.

Also if 9(2p−5)=72, why would you distribute the 9? I suspect that you know that 9*8=72, so why not just say that 2p-5 =8. Then 2p = 13 and finally p=13/2.


Your decimal answer of 6.5 is equal to 13/2.

 

[topsquark]

6.5 x 2 = 13 = 13/2 = 117/9

I agree with Jomo. I used to take points off for writing it this way. You really need to write out the steps on more than one line. Clarity is everything when a tired teacher is grading!

-Dan

 

[Steven G]

I agree with Jomo. I used to take points off for writing it this way. You really need to write out the steps on more than one line. Clarity is everything when a tired teacher is grading!

-Dan

Not being tired never bothered me from taking off points for invalid equal signs. However I do see how a tired teacher could grade more harshly.