David was not a great student - he did not always do all the problems on a homework assignment. Suppose a certain homework assignment had 10 problems, but David only did 4 of them. The grader will randomly select 5 of the 10 problems to grade.

(a)What is the probability that David did at least 1 of the 5 problems chosen for grading?

(b)What is the probability that David completed 3 or more of the problems chosen for grading?