Hi, I know the answer for 17 is E (because it is right-skewed) but stuck on 18.

The next two questions [17 and 18) refer to the following: The amount X spent (in $) by customers in the grocery store express lane follows some right-skewed distribution with mean $24 and standard deviation $15. 17. What is the probability that the average amount spent by the next three customers is more than $20? 18. What is the approximate probability that the next 40 customers spend less than $1,000 in total? (A) 0.5199 (B) 0.6064 (C) 0.6628 (D) 0.5784 (E) 0.6331