Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: if a and b are positive numbers , and a ^ 3 + b ^ 3 = 30 , then the greatest possible value of a is between :
3 and 4
3 and 5.