Prices are increasing at $3 \%$ per year. The following statement is incorrect. Provide the correct formula below for the percentage increase in prices over a decade. Prices change by $10 \cdot 3 \%=30 \%$ over a decade. NOTE: Enter the exact answer. The correct formula is $\%$.

Step 1 ：Given that prices are increasing at a rate of 3% per year, we are asked to find the percentage increase in prices over a decade.

Step 2 ：The initial statement suggests that the price change over a decade is simply 10 times the annual increase, which would be $10 \cdot 3 \% = 30 \%$. However, this is incorrect because it does not account for the compounding effect of the annual increases.

Step 3 ：The correct formula to calculate the percentage increase over a decade is \((1 + r)^t - 1\), where \(r\) is the annual increase rate and \(t\) is the time in years.

Step 4 ：Substituting the given values, \(r = 0.03\) and \(t = 10\), into the formula, we get \((1 + 0.03)^{10} - 1\).

Step 5 ：Calculating the above expression, we find that the percentage increase in prices over a decade is approximately 34.39%.

Step 6 ：Final Answer: The correct formula for the percentage increase in prices over a decade is \(\boxed{34.39 \%}\).