Doctor Audrey claims that the mean age at which children start walking is 12.5 months. The following data give the age at which 18 randomly selected children started walking. 15 11 13 14 15 12 15 17 10 16 14 16 13 15 15 14 11 13 Test at 5% level of significance if the mean age at which children start walking is different from 12.5 months