A percent comparison between two readings depends on which reading is the base. Because both battery readings use the same full battery as their scale, compare their numbers directly. Write the later reading as an unknown percent of the earlier one, then solve with a Desmos regression. Subtracting the readings instead gives a percentage-point drop, which answers a different question.
Hints
- Hint 1
The base is the amount a percent is taken of. In the phrase “of the charge level the night before,” which reading is the base for ?
- Hint 2
Both readings measure charge against the same full battery, so you can compare their numbers directly. Write the morning reading as of the earlier reading. Which reading belongs after “of”?
Step-by-step
Compare the morning charge with the earlier charge
Step 1Identify the base
Both battery readings measure charge against the same full battery, so you can compare with directly. The base is the amount a percent is taken of. The words “of the charge level the night before” make the earlier reading the base.
- Step 2
Turn the comparison into an equation
Translate “morning was of the charge level the night before” as morning = percent of earlier: . The applies to , not . Subtracting the readings would find the drop in percentage points, not the percent of the earlier charge still there.
- Step 3
Find the unknown percent
Type in Desmos. Here stands for , and tells Desmos to find the value that makes both sides match. Under PARAMETERS, Desmos shows . So the morning charge was of the charge level the night before. Choice C.