Answer
n = $(\frac{ z_α/_2 * σ}{E})^{2}$
When Confidence interval = 90%
α= 1-0.90 = 0.1.
α/2 = 0.05
1-0.05 = 0.95
From the table,
$z_α/_2$= 1.65
Since$z_α/_2$= 2.58 and E = 300 dollars, σ = 900 dollars
n = $(\frac{ z_α/_2 * σ}{E})^{2}$
= $[ $\frac{1.65 * 900 }{300}$ ] ^{2}$
=$(4.95)^{2}$
=24.5025= 25 sample
To be 90% confident that the true average amount of money a county spends on road repairs each year, 25 samples must be needed.