Answers:
For the first one the answer is six years
For the second one he earned $1273.94 from the interest and now has a total of $8773.94
Explanation:
Multiply 500 by 1.031 until you get 600 which take exactly six times
Then for the second one you multiply 7500 by 1.04 four times to get the total and subtract by 7500 to find how much he earned.
Hope this helps!