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A live opponent changes the incentive. Practising alone, there is nothing stopping a child from slowing down, guessing, or quietly stopping; with someone answering the same question at the same time, speed matters and attention holds. Scoring the first two correct answers rather than only the fastest keeps more players in the round after the opening seconds.

Mixed practice is harder than it looks, and usefully so. When every question is addition, a child can switch off and apply one procedure; when the operation changes each round, they have to read the problem first. That reading step is exactly what breaks down in word problems and on tests, so drilling it deliberately is worth the extra difficulty.