Official Explanation
Step 1: Understand the Prompt and Question
The diagram is not a standard chart, so you will definitely use the blurb to help you understand it. Before doing that, though, take the first step in SEE: Click the Select drop-downs to see what the answers are. Both are math focused. The answers for the first part of the question are in percentage form. They are far enough apart that you may be able to estimate at least enough to knock out some of the answers.
The answers for the second part of the question are in fraction form. The first two are clustered together and the final two are clustered together, so you may be able to eyeball enough to narrow down to two answers—that’s a valid strategy if it looks like it may take too long to fully solve.
Now, examine the text. The blurb first mentions two nonoverlapping categories, Vertebrates and Invertebrates. Glance at the diagram itself: These two categories appear at the bottom.
Nonoverlapping means that no animal appears in both categories. Further, the first sentence also states that these two categories together comprise all animal species, so all animals are either V or Inv. Abbreviate as you jot down this information.
The blurb further explains that the boxes above the V box represent subcategories of V and that the numbers represent the number (in thousands) of species. Glance at the diagram again. V’s are split into the categories Jawed or Jawless. Total, there are 66K V’s, most of which are jawed. Only a very small number are jawless. The jawed vertebrates can then be broken down into the categories Four-Limbed or Fish, and there are roughly equal numbers of each. Finally, the Four-Limbed group can be broken into four subgroups whose names appear only in the blurb: amphibians, reptiles, birds, and mammals. Reptiles and birds are the largest categories and mammals are the smallest category.
The question consists of two separate sentences. The first asks you to find Four-Limbed as a percent of Vertebrate. The second wants to know what fraction A’s and R’s represent of all species.
The two parts are not related, so you’ll solve each individually.
Step 2: Plan Your Approach
In each case, the task is to make sure that you are working with the right categories. (In general, if you see a nonstandard diagram that has a bunch of subcategories of subcategories, chances are good that at least part of your job will be to make sure you’re working with the right categories.) Organize your work carefully and double-check, before calculating, that you have pulled the data from the right groups.
Finally, keep an eye on the answer choices as you go; don’t do more math than you have to do.
Step 3: Solve the Problem
The first part asks for Four-Limbed as a percent of Vertebrates. The Four Limbed category has about 32.5K members and the Vertebrate category has about 66K members. Half of 66 is 33, so four-limbed species represent approximately 50% of all vertebrate species.
The second part asks for amphibians and reptiles as a fraction of all animal species. Jot down A + R for the top of the fraction. What about the bottom? Is that all vertebrates? Look back at the prompt.
No, it’s not just the vertebrates. The blurb says that vertebrates and invertebrates together make up all animal species, so you’ll need to add V and Inv. (Just double-check first that they didn’t already tell you the total in the prompt—but they didn’t.)
Total animals \(= 1,300K + 66K = 1,366K\)
That’s the bottom of the fraction. What goes on top?
\(A + R = 7K + 10K = 17K\) Here’s the full fraction:
Hmm. That’s much smaller than 1/2 or 1/4, so eliminate those two answers. But which of the other answers is it?
One way to find out: Plug that fraction into the calculator. The decimal is 0.0124. The decimal value of 1/100 is 0.01, and the decimal value of 1/200 is 0.02. The value 0.0124 is closer to 0.01, so the correct answer is 1/100.
If you’re not a fan of decimals, you can also simplify the fraction. The problem tells you that you can estimate and the two remaining answers are quite far apart, so you can estimate aggressively:
17/1366=200/1400=1/70
That fraction is closer to 1/100, so that’s the correct answer.
Here’s one more way! Work backwards from the two remaining answers. If 1/100 is the correct answer, then that fraction would approximately equal
17/1366. Does it? Use your calculator to find out (and do the same for the other remaining answer):
The fraction 1/100 is a closer match. Therefore, four-limbed species represent approximately 50% of all vertebrate species. Amphibians and reptiles combined comprise approximately 1/100 of all animal species.