When we say "miles per hour" in English this just is simply the question "If Bailey drove for only one hour, how far did she go?". Now mathematically we can find this very easily by simply dividing 290 by 4, which is equal to 72.5. So Bailey travels 72.5 miles every hour.
If it is confusing about why we do this, think about splitting up 290 miles evenly between 4 hours, how many miles does each hour 'get'. Each would 'get' 72.5 miles. So for every hour Bailey drives she travels 72.5 miles which is the same as saying that she drove at 72.5 miles per hour.
