Comparing OG PS Nr. 105 and 113

[sebastian.mauler]

Hello,

I have a question concerning the difference between Number 105 and 113 in PS, OG 13. In general, both tasks can be solved by using the unknwon multiplier, which i thought to be comfortbale with.

105 is absolutely obvios to me: I set up the equation: (30x+50)/(x+5)=25/1. This results in x=15. Because of the X beeing 1, concerning the teachers i get 1x15=15.
So the present number of teachers is 15.

I apllied the same logic for 113: I set up the equation: (3x+600)/(5x+500)=4/5. This yields x=200. For the present number of Republicans i get 3x200=600 and for Democrats i get 5x200=1000. Thus the difference should be, according to the same logic as in 105, 400 and not 300.
The video solution of MGMAT adds the new registered voters, before subtracting. 600 for the Reps and 500 for the Democrats. This yields: (1000+500)-(600+600)=300.

I do not understand why the 500 and 600 in 113 have to be added after getting the unknown multiplier. If this is true, why arent we forced to do the same in 105? We would have to add the increase of teachers of 5 and this would yield 15+5=20.

I am totally confused because of the different solutions, although both tasks seem similar to me.

Thanks for your support

Sebastian

[sebastian.mauler]

I Think i found my mistake. In 105 the increase is not a fact but an assumption. It says " ...were to increase". However in 113 it the increase is a fact. Is that right?

One last question: I will do the GMAT on 2nd April. I am studying since January. My target score is 600-650. Is it inevitable to do all the tasks of the OG? I get about 4 of 5 questions wrong since im doing the PS tasks with numbers 100+, which is really discouraging. Should i rather review older problems in the range 1-100 or keep trying new ones in the upper range? I fear to try problems i wont even face in the real exam and think it might be better to stick to the easier ones and focus on the basic problems. My last CAT was 630.

Thank you!

[tim]

unfortunately, we're not able to discuss OG13 problems here due to copyright issues. as for the second issue you brought up, that is something perhaps better suited for the general GMAT strategy section of our forum. in general, it is best to keep threads on this forum to one topic and to start a new thread if you are going to ask a completely unrelated question..