Seafood Buyers Why In The First Wuestion We Use The Z-distribution And In The Second The T-distribution?

Why in the first wuestion we use the Z-distribution and in the second the t-distribution? - seafood buyers

A shrimp buyer goes to a wholesale seafood and controls a large catch of shrimp over 50,000. It has the task of buying, if the average weight of 55 g shrimp. A sample of 60 shrimp were and are weighted. The average weight is 56.2 grams with a standard deviation of 4.2 g. If there is sufficient evidence at a level of 5% to reject the trick?

Fabtread motorcycle tires produced. Under normal test cycle average downtime motor trips to 60km / h is 3.12 seconds. The production team recently designed and developed a new tread on the tires. Under normal test has taken 41 measures of downtime and found that was the average time 3.03 seconds with a standard deviation of 0.27. Is there enough evidence at a level of 1% to support the belief that the team has improved the downtime?

We know that both the standard deviation. Why not also sitribution the Z?