Some numbers do not have an exact cube root. You then have to estimate a value, based on known cube roots that are less or greater than the given value.
27 < 38 < 64
The answer is 3 because 38 is closer to 27 than it is to 64.
You can work out that the answer has to be a value between 3 and 4, because the cube root of 27 is 3 and the cube root of 64 is 4. The answer will also be closer to 3 than to 4, because 32 is closer to 27 than it is to 64.