Chinese Students Have To Solve This Question For Wi-Fi Password

Solve the questions

When the students try to solve the questions in some different way as taught by the teacher from prescribed books ,then these students should be :

In an examination of n questions, a student replied 15 out of the first 20 questions correctly.of the remaining questions, he answered one-third correctly. All the questions have the same credit. If the student gets 50% marks, the value of n is a.20 b. 40 c. 50 d. 100

The probabilities that a student will solve Question A and Question B are 0.4 and 0.5 respectively. What is the probability that he solves at least one of the two questions ?