Thanks for the clarification. I can understand that solution is provided considering probability of studying CS on monday is 1

My concern is how do we decide that ? This phrase is “either computer science or mathematics everyday” is bit ambiguous. Does it mean on a particular day it can be either CS (or) maths but not both as its given as “either-or” ?

If thats the case, then in what scenario we can assume that P(CS) = 0.5 ? Usually in toss of coin when question says outcome can be either head (or) tail, we do a similar assumption that P(H) = P(T) = 0.5. So, wanted to know why a similar approach cant be applied here