Question

why 145.4 can be written as 145.20 please explain but no spam otherwise you will be reported​

Answer 1

"90% of the mails it marks as spam are indeed spam" -- this part of the question should be a misprint as it gives the asked answer as 90% but this is not consistent with the other data given. Avoiding this part we can solve this question as follows:

10% email are spam, i.e. 90% email are not spam 

90% of mail marked as spam is spam, 10% mail marked as spam are not spam 

By Bayes theorem the probability that a mail marked spam is really a spam 

=Probability of being spam and being detected as spamProbability of being detected as spam=0.1×0.9(0.1×0.9)+(0.9×0.1)=50%