CFAI Quant - 99% confidence interval

This question has been bugging me. The question gives you a 99% confidence interval of $230,000 and $480,000 and asks you to compute the mean and standard deviation. Answer: mean of $355,000 and standard deviation of $41,667 Simple enough but I can’t figure out why they divide by 3 to find the standard deviation - shouldn’t it be +/-2.58 giving a standard deviation of $48,449? Did they simply just round up to 3 or is there something that I’m missing here.

rounded up to 3.

i know it sounds weird, but agree with cpk