Computation of z or t value of the sample test statistic.?

the av.calories in a serving of popcorn is 75. To determine if the average number of calories is different from 75, a nutritionist selected a random sample of 20 servings of popcorn and computed the sample mean number of calories per serving to be 78 with a sample standard deviation of 7. Compute z or t? At the .05 level of significance, can we reject the writer' s claim/