standard deviation questions (10 points)?

hi..im not sure how to do standard deviation plz help. dont worry if you cant answer them all. thanks.

1.A country town instals 2000 new electric lights in a new housing estate. These lamps have an average life of 1000 hours with a standard deviation of 200 hours.
a) What percentage of bulbs would be expected to fail between 800 hours and 1200 hours? How many bulbs would this be?
b) How many bulbs would be expected to last longer than 1600 hours?

2.In a kindergarten class of 40 children the average foot length was calculated to be 10.5cm with a standard deviation of 0.5cm.
a) what percentage of lengths are
i) between 10cm and 11cm?
ii)less than 10cm?
iii)more than 11.5cm?
iv) between 9cm and 12cm?
b) How many children (nearest whole number) have foot lengths;
i) between 10cm and 11cm?
ii) less than 10cm?

3.A manufacturer of zinc plated nails manufactures a certain type of nail of mean length 3.8cm, with standard deciation 0.02cm. Gauges reject all nails longer than 3.84cm or shorter than 3.76cm. WHat percentage of nails are rejected? In a run of 10 000 nails , how many (nearest whole number) would you expect to be rejected? If one nail is selected at random, what is the probability that it would NOT be rejected?

4.Consider the set of five numbers 8, 3, 7, 4, 3.
a) Add five to each number in the group. How does this change the mean and standard deviation?
b) Multiply each of the numbers by five. How does this alter the mean and standard deviation?

thank you for helping if you can .