Why is the formula for blackhole entropy different that the formula for entropy given by Boltzmann? why are there different formulas for entropy? (the Boltzmann one and the black hole one given by hawking). 
entropy by Boltzmann is $S= k \log W$
entropy for a black hole is $S=zA$ where $A$ is the are of the event horizon
 A: Both definitions of entropy that you've mentioned are just special cases of the general concept of entropy, applied to different systems.
The general concept of entropy is defined for a macrostate, which is just a probability measure $p$ over microstates. Entropy is defined as:
$$
S = k\sum_x p(x) \log p(x)
$$
Up to choice of units, where the sum is taken over all microstates. In the case of the Boltzmann entropy, we are dealing with an ideal gas, and the assumption is that all microstates are equally probable, so the entropy reduces to just being proportional to the log of the number of microstates:
$$
S = k\sum_x p \log p = -k\sum_x \frac{1}{W} \log W = k\log W
$$
Deriving the entropy of a black hole is non-trivial; this description (section 3) might be of use in understanding it. The scholarpedia page on black hole entropy also has some useful links. The hand-wavy explanation is as follows. For a non-charged, non-rotating black hole, the macrostate observable is the mass, and an external observer looking at the black hole can only see as far as the event horizon. A microstate thus corresponds to a particular configuration of the event horizon, so the number of possible microstates is proportional to $\exp A$ where A is the area of the event horizon in some choice of units. Thus the entropy is just proportional to $A$.
