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All of these questions have been discussed throughly in this section of the forum in less than 6 months. Please take some time searching the previously posted GMATprep posts and you'll find more than what you're looking for.
question 3: i loved this question....tricky..here's the solution
for this question,simply assume that proportionality implies equality......
(the conceptual logic behind this is that in problems about percentages and proportions,the equations must hold true for any value...therefore you can assume the proportionality constant to be 1...)
u get the following equations from the questions...
r=a^2...where r=rate of chemical reaction, a= concentration of chemical A,b=concentration of chemical B....
now as per the question, b is increased by 100%...i.e doubled....
understand that in the second equation,if b increases,the value of the right hand side of the equation decreases.To compensate for it, the value of 'r' must INCREASE since the two are inversely proportional.
therefore, an increase of 100% in 'r' as well...this makes the LHS = 2r...now coming back to chemical A.....if r=a^2,then 2r= 2 * (a^2)..
i.e (root 2 * a)^2....therefore 'a' becomes (root 2 *a)..i.e 1.41 * a...an increase of 40 %.....
for this question,instinctively i would probably work backwards
my 2 cents on the search function on phpBB boards. Somehow the search function on these boards (the actual interfase) doesn't work quite well and often results are really bad. I've seen it here and in a few other phpBB powered forums.
I think they should, instead, have a 3rd company take care of the search engine, like google.