The weights of ripe watermelons grown at Mr. Smith's farm are normally distributed with a standard deviation of 2 lb. Find the mean weight of Mr. Smith's ripe watermelons if only 5% weigh less than 18 lb. (Give your answer correct to one decimal place.)

Question

The weights of ripe watermelons grown at Mr. Smith's farm are normally distributed with a standard deviation of 2 lb. Find the mean weight of Mr. Smith's ripe watermelons if only 5% weigh less than 18 lb. (Give your answer correct to one decimal place.) 
 Incorrect: Your answer is incorrect. . 8.8 lb 
I have tried this one several times and can not get it right, please help!!!!!

Steve · Answer

.05 is P(Z<-1.645) That is, 18 is 1.645 std below the mean. So, the mean is 18+2*1.645 = 21.29