At the archery training ground, some archers shoot arrows at targets at a distance of between 20 meters and 70 meters. The probability of hitting a target with one shot is p. When shooting from a distance d meter, p = 0.015 (70 - d). Each arrow is released independently.

1.Can you explain why this is a binomial probability distribution case, assuming y shots are taken? (5 points)

2.An archer releases 10 arrows from a distance of 40 meters.

a.Find the probability that exactly 4 arrows hit the target! (5 points)

b.Find the probability that at least 8 arrows hit the target! (5 points)

3.Another archer releases 20 arrows from a distance of x meter, find the value of x if the archer has an 80% chance of hitting the target at least once! (10 points)