Asample size of n=12 does not have a known population standard deviation. the population appears to be normally distributed. determine whether a margin of error should be calculated using a critical value of z a/2, a critical value of 1 a/2, or neither

Expresa commuter train leaves downtown sation and travels at a average speed of 55 miles per hour towards the north side sation, which is 50miles away. thirty minutes later, express commuter train #7 leaves north side and travels at a average speed of 35miles per hour towards downtown sation. at the moment the two trains pass each other, how far(in miles) is train #12 from the downtown sation and how long(in minutes) has the #12 train been traveling