4. (Total 15 points) The time (in hour) for students to complete an exercise is a random variable X. The pdf of X is f(x)={cx2+x,0,0x0.5otherwise (a) (4 points) Find the value of c. (b) (4 points) Find the cdf of X. (c) (3 points) Find the probability of completing an exercise in 20 minutes. (d) (4 points) Find the probability of completing an exercise in more than 10 minutes