An express train and car leave los angeles at the same time and head for merced 280 miles away. the rate of the train is twice the rate of the car. the train arrives 4 hour before the car arrives. find the rate of the car.

Now, recall your d = rt, or distance = rate * time

so.. the train and the car, both leave LA, and go to Merced, which is 280miles away.....alrite, the train is going twice as fast as the car is, so, if the car is going "r" fast, then the train is going 2*r or 2r fast

now, the train arrives 4hrs earlier than the car, so, if the car took "t" time to arrive, the train took then t-4

now, both, arrived at Merced, that means, the distance covered by each, is 280miles

thus      

solve for "r"

C)He should also take a sample from each of the other history classes in all of the other grades in the middle school.

Step-by-step explanation:

Not only students in the 8th grade are in the middle school, there are other students in other grades who are in the middle school and are enrolled in history classes. Leo's sample is only from one grade of the middle school. In fact, including Ms. Lopez's and Mrs. Frank's classes in the selection of sample is not sufficient. Leo has to take a sample from the remaining grades to avoid making a wrong conclusion as a result of bias in the choice of data.