Let's start with problem #1, which will give you a underlying knowledge for the other problems.
You have a number set of time scores as follows:
24, 18, 21, 17, 25, 19, 23, 17, 25, 15
To get standard deviation, you need variance, to get variance, you need mean. Go here, and enter your number set:
Mean, Variance, and Standard Deviation
Follow the math to see how to calculate mean, variance, and standard deviation.
Then we can move on to the other problems.