Answer
See below.
Work Step by Step
We know that $probability=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}$
a) The number of good outcomes is $243$
The number of all outcomes is $243+26=269$
Hence the probability here is: $\frac{243}{269}$
b) The number of good outcomes is $13$
The number of all outcomes is $13+18=31$
Hence the probability here is: $\frac{18}{31}$
