mba4me wrote:

M is a positive integer, How many digits does M^3 have?

1) M is 3 digit number

2) M^2 is 5 digit number.

Answer is E

1) not sufficient...could be anything 100^3 = 1,000,000 (using the minimum 3 digit number), and 1000^3(easier than using maximum 3 digit number of 999) = 1,000,000,000

2) not sufficient...sq.root 10,000 (minimum 5 digit number) = 100, so bump it up a bit to 300 and 300^2 = 90,000 (close to the maximum 5 digit number), but still don't know how many digits m^3 has, because 100^3 = 1,000,000 and 300^3 = 27,000,000

Combined) still not sufficient