Skimming answers "what is this about?"
Skimming means reading the title, first and last sentence of each paragraph, and any headings, to build a rough map of the whole text in under a minute. You are not trying to understand every sentence — you are trying to know where each idea lives, so that when a question asks about it later, you already know which paragraph to return to.
Scanning answers "where exactly is this detail?"
Scanning means moving your eyes across the text looking for one specific thing — a name, a date, a number, a keyword from the question — while ignoring everything else. You are not reading for meaning at all; you are pattern-matching for a shape on the page.
Why readers who "read carefully" run out of time
If you read every word at full comprehension speed on a first pass, you spend your entire time budget before you even know which paragraphs matter. Skim first to build the map, then scan or read closely only the parts the questions actually need.
Practice
Take any two-page article. Give yourself 60 seconds to skim it and write one sentence summarising each paragraph from memory. Then pick three specific facts (a number, a name, a date) and time how fast you can scan back to find each one.