I think 'they' expect you to just learn these two facts.
68% lie between mean - 1 sd and mean + 1 sd
So you get 100-16 to 100+16
95% lie between mean -2 sd and mean + 2 sd
So you get 100-32 to 100+32
The mean is 100. Standard Deviation is 16 on a normal curve.
The question is:
If the distribution of mental ability scores is normal, the middle 68 percent of the scores should lie between what two scores?
answer: 84 to 116. How do you find this?
The next question is:
The middle 96 percent of the mental ability scores should be between what two values?
answer: 68 to 132
Its killing me.
Maybe the book used a method that:
1. Gets a more accurate answer, and
2. Provides a template for answering questions like
"If the distribution of mental ability scores is normal, the middle [40] percent of the scores should lie between what two scores?"
You should realise that
"68% lie between mean - 1 sd and mean + 1 sd"
is a rough rule of thumb ("68-95-99.7 rule") - useful under certain circumstances.
Hey, it's no big deal. Where I was coming from is that sometimes a student gets an easy question that can be done using an easy method (often just a memorised rule), but when the question gets a bit trickier (like the example I gave) the student flounders becuase they have no general process to fall back on.
Great to have you on board, by the way