For doing a test for the mean of a normal distributed variable in R do:

t.test(variable, mu=55)

mu= 55 is the value of the meaning that you want to test


The output is something like:

One Sample t-test

data: variable
t = -1.3736, df = 145, p-value = 0.1717
alternative hypothesis: true mean is not equal to 55
95 percent confidence interval:
49.82150 55.93193
sample estimates:
mean of x

In this case the value being tested for the variable mean is inside the confidence interval of the mean of the variable.