what is the youngest this person can be? Martha said: “I have been alive for all or part of five decades.” Rounded to the nearest year, what is the youngest she could be? How can this be solved?

The answer is 30.

If someone was born December 30, 1979 and passed Jan 2nd, 2010 then they will have lived 30 years and in part of 5 decades (with e.g., the 70s beginning in Jan 1st 1970 and ending Dec 31st, 1979).