Compound percentage reduction can be calculated by multiplying the initial number by (1-x)^t where x is the percentage reduction (/100, i.e. 1% reduction is 0.01), and t is the number of times it has reduced.
So: a) Where t is the number of weeks.
We can then just substitute in to work out the final pressure: b) p = 45*0.94^6 = 31psi