Calculate the Exact Number of Parks with 24 Trees Planted

To solve this problem, let's calculate step by step:

• Determine the total number of parks: Since all parks combined plant an average of 24.2 trees each, let's assume there are $n$ parks. This implies a total of $24.2n$ trees.
• Calculate trees in known parks: The first two parks have 19 trees each, thus total $2 \times 19 = 38$ trees. The next three parks have 28 trees each, amounting to $3 \times 28 = 84$ trees. Together, they plant $38 + 84 = 122$ trees.
• Set up the equation: Let $x$ be the number of parks that planted 24 trees. Then the rest of the parks account for $24x$ trees. Our equation becomes:

$24.2n = 122 + 24x$

• Express $n$ in terms of $x$:
• Since $n = 2 + 3 + x$ (two parks with 19 trees, three with 28, and the ones we are solving for), we substitute this into the equation:

$24.2(2 + 3 + x) = 122 + 24x$.

• Solve for $x$:

$24.2 \times 5 + 24.2x = 122 + 24x$ $121 + 24.2x = 122 + 24x$ $121 + 24.2x - 24x = 122$ $0.2x + 121 = 122$ $0.2x = 1$ $x = \frac{1}{0.2} = 5$

Therefore, $\boxed{5}$ parks planted exactly 24 trees. This confirms with answer choice (3).