hypothesis and standardized test statistic

had to miss my stat class the other day and i'm having trouble on my homework. i got a little but any step by step guidance would be appreciated

The Department of Economic and Community Development (DECD) reported that in 2009 the

average number of new jobs created per county was 450. The department also provided the following

information regarding a sample of 5 counties in 2010.

County New Jobs Created

In 2010

Bradley 410

Rhea 480

Marion 407

Grundy 428

Sequatchie 400

a. Compute the sample average and the standard deviation for 2010.

b. We want to determine whether there has been a significant decrease in the average number

of jobs created. Provide the null and the alternative hypotheses.

c. Compute the standardized test statistic. Assume the population is normally distributed.

d. At 5% (α = .05) significance, test the hypotheses. State your conclusion.

a) mean = 410 + 480 +407 + 428 + 400 / 5

Deviation = sqrt[(410-425)^2 + (480-425)^2 + (407-425)^2 + (428-425)^2 + (400-425)^2 / 4]

= 32.43

...and i'm lost now