Dementia is the loss of the intellectual and social abilities severe enough to interfere with judgement, behavior, and daily functioning

Dementia is the loss of the intellectual and social abilities severe enough to interfere with judgement, behavior, and daily functioning. Alzheimer's disease is the most common type of dementia. A sample of 21 people with early-onset dementia gave the following data on age at diagnosis. 60 53 58 44 52 46 58 47 59 42 58 56 51 57 61 49 54 41 59 43 55 At 1% significance level, do the data provide sufficient evidence to conclude that the mean age at diagnosis of all people with early-onset dementia is less than 55 years old? Assume that the population standard deviation is 6.8 years