Probability of passing history course = 0.59
Probability of passing math course = 0.60
Probability of passing both courses = 0.49
Probability of passing at least one = 0.59 + 0.60 - 0.49 = 0.7 ##
199.
the z score for the 90th percentile is 1.28, so you can solve the equation

which gets you the 199
Inverse relationships are of the form xy=k or if you prefer y=k/x so to find the constant we can say:
18(0.25)=k=4.5
So you could say: IR=4.5 or I=4.5/R