Why is it important to have categories of numbers and to know which categories you are dealing with in a given expression?

Gabrielle's age is two times mikhail's age. the sum of their ages is 84. what is mikhail's age?

Randy and trey take turns cleaning offices on the weekends. it takes randy at most 4 hours to clean the offices. it takes trey at most 6 hours to clean the offices. what is the greatest amount of time it would take them to clean the offices together? 2.4 hours 2.5 hours

597008.439431 to the nearest thousand