ACT Preparation / Unit 19 ACT 1816 worked example, 60 to 90 seconds
Extrapolating a Trend
The idea
A value beyond the data is estimated in three steps. First, find the pattern in the last few measured points: a steady difference, a steady ratio, or a leveling off. Second, continue that pattern to the condition the question names. Third, choose the answer consistent with the pattern, remembering that the estimate depends on the pattern holding.
In practice
Worked example: an invented table shows a culture's count, in thousands, at hours 0, 1, 2 and 3: 5, 10, 20 and 40. The count doubles each hour, so at hour 5 it would be 40 × 2 × 2 = 160 thousand, if doubling continues. A choice of 80, which keeps adding the last difference of 20, mistakes the doubling for a steady difference.
Where people slip
Decide whether the pattern adds or multiplies before extending it; the two diverge quickly beyond the data.
Your place on the trail
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