This question is for very basic understanding of the Standard Deviation. You should not be struggling with this. It is a standard z-score.
Mean = 50
Standard Deviation = 10
Between 40 and 60
Recognize this immediately as one standard deviation on each side of the Mean. There is an Empirical Rule for that, 68-95-99.7 rule - Wikipedia, the free encyclopedia, making it an eyeball problem, but only if you know the rule.
Between 30 and 70
Recognize this immediately as two standard deviations on each side of the Mean. Please consult the Empirical Rule for an instant answer.