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18) The auto insurance company usually pays the claim by two parts: body injured and property damage. The cost for body injured follows a normal distribution with mean 500 and standard deviation 20. The cost for property damage follows another normal distribution with mean 200 and standard deviation 10. These two costs are independent. Assume that the total cost of two parts also follow normal distribution. What is the number x that the probability of total cost of the claim is less than x will be 0.9?