I'm sorry for the tone of my last sentence; my intention was only to let you know how to ask questions better in the future, and I was responding to your very nice expression of thanks and respect, not to any disrespect I perceived.
The more you tell us about the problem and your needs (as you have now), the better we can do. (If I were teaching a child, I would start out by trying to find out what she knew and encouraging her to try as much as she could, not by assuming she knew nothing). The most important thing in helping a student is knowledge of their particular needs.) I'll try to treat you as a
beginner, which you are.
Okay, somewhere (maybe not in this problem, but in the header for a set of problems), it must have said to assume a normal distribution. That's an essential part, and is the reason the first response you got was totally unrelated to your actual need.
So the problem was,
Mean=100
s=20
What percentage of people scored between 100 and 110?
You are expected to sketch the normal distribution, where the middle (mean) will be 100. Then you would draw vertical lines at 100 (the middle) and at 110 (somewhere to the right of that) and shading in the region between. The percentage you want can be described as the area of the shaded region.
The first step is to find z, by plugging the given numbers into the formula. You know the mean and standard deviation; the x is any value you are interested in, in this case 110.
So the value of z corresponding to x=110 is (110 - 100)/20. Calculate that.
Now what to do next depends on some things I asked about that you have not answered, probably because you don't know why I had to ask. How are you finding the area (probability) in your course? In some courses, you are given a computer program to do this, or use a graphing calculator. In others, you are given a table to look it up in. But even those tables differ. Some give the answer to your question directly (the area between the middle and a given value); others give the entire area to the left, and for your problem you would have to subtract 50% (the left half of the graph). So this is one of those situations in which I would much rather be sitting next to you, so I could look over your book or notes and determine what method you need to use! You can see examples of three kinds of tables
here. Please let me know what your class is using, so we can continue.