Data Sufficiency

The only contents of a parcel are 25
photographs and 30 negatives. What
is the total weight, in ounces, of the
parcel's contents?
(1) The weight of each photograph is
3 times the weight of each
negative.
(2) The total weight of 1 of the
photographs and 2 of the
negatives is
1/3 ounce.

OA is C

grandh01 wrote:The only contents of a parcel are 25
photographs and 30 negatives. What
is the total weight, in ounces, of the
parcel's contents?
(1) The weight of each photograph is
3 times the weight of each
negative.
(2) The total weight of 1 of the
photographs and 2 of the
negatives is
1/3 ounce.

For questions like these (where you are given an equation with to variables) its important to know what you need. What we learned back in Jr. High is that for 2 variables we need two equations, but the GMAT likes to trick us. Sometimes you only need 1, and sometimes 2 doesn't work. So how to avoid the trap? So ask you self:

1. how can I solve this with only 1 equation? You can do this if you can manipulate the equation to get the information you are looking or, even if you don't solve for each individual variable. For an easy example if they ask: What is the value of x+y? and they give you 2x+2y=6. This is sufficient because you can divide everything by 2 and get x+y=3. Obviously this is way too easy and the GMAT will ask it more difficultly, but remember you can solve it with only one equation.

2. How can I not solve even with two equations? This happens when both equations are the same (although manipulated). For example what is the value of P+H. 1. 2p+3h=12.5. 2. 4p+6h = 25.00. The answer to this is E because both equations are the same.

Now... to the problem at hand...
Here we have an equation needs to be solved what is 25p+30n?

Statement 1: p=3n. Ask question 1? No we cannot, insufficient.
Statement 2: p+2n=1/3. Ask question 1? No we cannot, insufficient.
Together: Two equations, two variables. Can we solve? Ask question 2, are they the same? No they are not so we can solve. Correct answer is C

Let me know if this helps.

IMO C

Three variables. Need three equations to solve.