Normal distribution (NF)
A standard method from Measures of dispersion: the steps.
Steps
About 68% of data lies within 1 SD of the mean, 95% within 2 SD, and 99.7% within 3 SD. The question gives the percentages needed.
Want to check an answer? Standard deviation calculator shows every step.
For the whole topic, read Measures of dispersion: key points, common mistakes and how the DSE tests it.