A school has 40 students. In how many ways can 30 of the students be chosen to form a class?

Can someone me with this problem i don't understand i need

Maria walks a round trip of 0.75 mile to school every day.how many miles will she walk in 4.5 days

Cody started biking to the park traveling 15 mph, after some time the bike got a flat so cody walked the rest of the way, traveling 4 mph. if the total trip to the park took 6 hours and it was 46 miles away, how long did cody travel at each speed? solve using rate times time= distance