P(x>1250): 1250 is the mean, so it's in the middle of the distribution curve. The portion to the right of the mean is the portion which is greater than 1250. Therefore, 50% is greater than 1250.
They want the area in between 1250 and 1380. So, using the formula, we find . Look up 0 in the z-table. It corresponds to 0.50, as we seen in the previous problem--because it's on the mean. That's why the z-score is 0; The z-score is the number of standard deviations from the mean; So, 1380-1250=130. 120 is 1 standard deviation and this is 130. just a little more. It's 1.08333.....See?. Therefore, it's 1.0833 standard deviations from the mean.
Using the formula and the table, we see that z=1.08 has a prob. of 0.86(in the body of the table).
0.86-0.50=0.36. 36% is between 1250 and 1380.
You do the last one. Okey-doke?.
As far as the first problem goes, are you missing some info?. Say, maybe the number in the sample?