To solve the problem using unit rates, we first determine the rate at which Peter prepares the dough.
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Find the rate of dough preparation: Peter prepared 15 kilograms of dough in 3 hours. We can calculate the rate of dough preparation in kilograms per hour:
\[ \text{Rate} = \frac{\text{Kilograms of dough}}{\text{Hours}} = \frac{15 \text{ kg}}{3 \text{ hours}} = 5 \text{ kg/hour} \]
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Determine the time taken for 25 kilograms of dough: Now that we know Peter can prepare 5 kilograms of dough in 1 hour, we need to find out how many hours it will take him to prepare 25 kilograms:
\[ \text{Hours} = \frac{\text{Kilograms of dough}}{\text{Rate}} = \frac{25 \text{ kg}}{5 \text{ kg/hour}} = 5 \text{ hours} \]
Therefore, Peter worked 5 hours to prepare 25 kilograms of dough.