Approximate values of these logs?

given that log_b 2=(approximately)0.756 and that log_b 10=(approximately)2.513, find the approximate values of each of ther following

a) log_b 40

for this one I got log_b 2^2x10. (x is times) then I got (.756)^2x10 =5.715. I have no idea if this is right? I am also unsure of..

b)log_b 1/2

I know I have to flip the fraction, so would that make it -log_b 2?

c) log_b 81/log_b 27

no idea what to do with this one?

d)log_b 1/b^3

same here. i just don't these.

there are a few more to this problem, but I got those ok. these have me stumped??