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SAT Reading and Writing Practice
Central Ideas and Details

Identify the main idea of a passage and distinguish it from supporting details.

66 original central ideas and details questions in the SAT Ranker bank, with instant scoring and an Elo-based rank once you sign up. Below are a few free samples with full explanations.

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Sample question 1 · easy

The first traffic lights were installed before cars were common. They were built in London in 1868 to control horse-drawn traffic near Parliament, and they were operated by a constable turning a handle. The idea was sound, but the gas that lit them exploded within a year, and the experiment was abandoned.

Which choice best states the main idea of the text?

The first traffic signals in London were operated by hand rather than automatically by an operator on the spot.
✓ Traffic lights were first used for horses and were abandoned after the gas lighting exploded.
London had serious traffic problems near Parliament well before cars arrived.
Gas lighting proved to be unreliable in many Victorian public installations.

The text says the first lights predated cars, describes their use on horse traffic, and ends with the explosion that stopped the experiment. That span is the main idea. The hand operation is a detail within it, and the text makes no wider claim about Victorian gas lighting.

Sample question 2 · easy

Paper money began as a receipt. Merchants in Tang China left heavy strings of coins with trusted agents and carried away a written note promising the same amount elsewhere. The note was easier to move than the coins, and in time people began trading the notes themselves rather than redeeming them.

Which choice best states the main idea of the text?

Coins used in Tang China were too heavy for merchants to carry over long distances.
✓ Paper money grew out of receipts for deposited coins that people began trading directly.
Merchants in Tang China relied on trusted agents to hold their money securely.
Written notes were more difficult to counterfeit than the coins they represented.

The text traces notes from receipts for deposited coins to objects traded in their own right, which is how paper money began. The weight of the coins and the role of the agents are steps in that account, and counterfeiting is never mentioned.

Sample question 3 · hard

Passage 1: Proponents of remote work argue that eliminating commutes gives employees more time for rest and family, boosting long-term productivity and reducing burnout. Passage 2: Critics counter that in-person collaboration accelerates the kind of spontaneous problem-solving that structured video calls rarely replicate, and that many employees report feeling more isolated when working remotely full-time.

Based on both passages, a researcher studying workplace productivity would most likely conclude that

remote work is universally more productive than in-person work.
✓ the productivity effects of remote work involve trade-offs between reduced burnout and reduced spontaneous collaboration.
in-person work has no benefits that remote work cannot replicate.
employee isolation has little connection to how much of a person's working time is actually spent away from a shared office.

Passage 1 highlights a benefit of remote work (reduced burnout) while Passage 2 highlights a drawback (reduced collaboration, isolation). Together they support a trade-off conclusion, not a one-sided one.

Sample question 4 · easy

Trees in a forest are not entirely separate. Their roots are linked by fungal threads that carry water and nutrients between them, and a large tree in good light may pass sugars to a shaded seedling of the same species. The forest behaves in part like a single connected system.

Which choice best states the main idea of the text?

Fungal threads in forest soil are far more extensive than was once believed.
Seedlings growing in shade struggle to produce enough sugars of their own.
Large trees compete with smaller ones for the light available in a forest.
✓ Trees in a forest are connected by fungi and share resources through them.

The text says trees are not separate, describes the fungal links that move water, nutrients and sugars between them, and concludes that the forest acts as a connected system. The shaded seedling is an example of that sharing, and the text describes cooperation rather than competition.

Sample question 5 · medium

The convention that a scientific paper reports work in the order it was done is a useful fiction. Hypotheses are frequently formed after the data arrive, dead ends are cut, and the tidy sequence of question, method, result exists to let a reader check the argument, not to record the week. Treating the form as a diary produces a false picture of how discovery happens, and an unfair one of scientists who describe their own work honestly.

Which choice best states the main idea of the text?

Scientific papers ought to be rewritten so that they record the actual order in which the work was done including every dead end that the researchers ran into along the way.
Scientists frequently omit the dead ends they ran into from their published accounts.
Most hypotheses in published science are formed only after the data have been collected.
✓ The structure of a paper serves the checking of arguments, not the recording of events, and mistaking the two distorts how discovery is understood.

The text calls the ordering a useful fiction, says the sequence exists to let a reader check the argument rather than to record the week, and warns that reading it as a diary distorts how discovery is understood. That is the main idea. Late hypotheses and cut dead ends are the evidence, and the text recommends no rewriting.

More topics
SAT Reading and Writing skills
Words in ContextCommand of EvidenceInferencesText Structure and PurposeCross-Text ConnectionsRhetorical SynthesisTransitionsBoundariesForm, Structure, and Sense