A guy bikes 15 miles in 1 hour, then rests for an hour. Then he bikes 25 in 2 hours. What was his average speed for the trip?

He went 15 miles/hour in the first sequence, and then 12.5 miles/hour (25 miles/2 hours) in the second sequence. If you take into account the hour he rested between segments: 15 miles/hour, 0 miles/hour, and 12.5 miles/hour. So (15+0+12.5)/3 = 9.17 miles/hour is his average. However, if you are not taking into account the whole trip, and only the time he was biking: (15+12.5)/2 = 13.75 miles/hour

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.