Question 192·200 Super-Hard SAT Reading Questions·Information and Ideas
A microbiologist prepared three bacterial cultures, each containing the same six strains. Each culture initially contained 120 million cells—20 million from each strain—including strain R. After 48 hours, the microbiologist recorded the total cell counts shown in the table.
| Nutrient treatment | Initial cell count (millions) | Final cell count (millions) |
|---|---|---|
| No added nutrient | 120 | 132 |
| Ammonium | 120 | 200 |
| Nitrate | 120 | 220 |
Strain-level analysis indicated that strain R accounted for 70% of the nitrate culture's net increase. In the ammonium culture, no individual strain accounted for more than 25% of that culture's net increase. The strains' individual changes in the culture with no added nutrient were not recorded.
Which choice is best supported by the text and table?
For questions combining a table with percentage descriptions, identify exactly what each percentage measures. Calculate the relevant change from the table, apply the percentage to that change, and then account for any initial amount. Pay close attention to upper bounds such as “no more than,” since they can support a comparison without determining an exact value.
Hints
Find each net increase
Subtract each culture's initial total from its final total before applying the percentages.
Use the correct denominator
The percentages describe each culture's net increase, not its final cell count.
Account for the starting cells
After finding strain R's increase, add the 20 million strain R cells present initially. For the ammonium culture, determine an upper bound rather than an exact count.
Step-by-step Explanation
Calculate the cultures' net increases
The nitrate culture increased from 120 million to 220 million cells, a net increase of 100 million. The ammonium culture increased from 120 million to 200 million cells, a net increase of 80 million.
Determine the strain R counts
Strain R accounted for 70% of the nitrate culture's 100-million-cell increase, so it gained 70 million cells. Adding its initial 20 million cells gives a final count of 90 million.
In the ammonium culture, strain R could account for no more than 25% of the 80-million-cell increase, or 20 million cells. Thus, its final count could be no more than 40 million.
Compare the final counts
Twice the ammonium culture's maximum possible strain R count is 80 million, which is still less than the nitrate culture's 90 million. Therefore, the answer is At the end of the experiment, the nitrate culture contained at least twice as many strain R cells as the ammonium culture did.