Assigning SMART NUMBERS

[alexisvargas85]

Sorry if I did not write the question completed, but I sent an email asking if I have to do so, and a person from Manhattan told me it was not necessary since all the instructors have the online version of all the Strategy Guides. Anyways I am writing the entire question below:

I have a question in the Fractions, Decimals & Percents Strategy Guide, Manhattan PREP, fifth edition.

In the page 120, the 10th the problem of the Problem Set (chapter 7) says:

In a grocery store sells two varieties of jellybeans jars, and each type of jellybean contains only red and yellow beans. If Jar B contains 20% more red jellybeans than Jar A, but 10% fewer yellow jellybeans, and Jar A contains twice as many red jellybeans as yellow jellybeans, by what percent is the number of jellybeans in Jar B larger than the number of jellybeans in Jar A?

In this solution they assigned as a smart number A=300 beans, from which red are 200 and yellow are 100. However, when I was solving by myself I assigned 300 too, but red: 100 and yellow: 200. When I solved the problem it resulted that the percent change was 0. My answer was 0 percent of difference between A and B and the right answer was 30%. I did not make any mistake operating but probably it was a problem assigning a good smart number, but how can I think about a "good smart number" that works for a problem? I do not see exactly what I did wrong in this problem
I would like to know some criteria that allow me to not make these mistakes since I will have just 2 min to solve the problems, and resolving or reassigning smart numbers can be time-consuming.
Thank you,
Alexis

[RonPurewal]

Hi Alexis,
Could you please post the full text of the problem (and, if appropriate, the parts of the solution that you're quoting)? We don't always have access to the guides when we are posting -- and we'd like to create threads that will be useful to other readers of the forum.

Thank you.