1.   If you're sure what you're asking is "rate" of acceleration then it is NOT changing. Rate of change means the quantity must be changing with time.

The acceleration due to gravity (g) during free fall is always constant. It has a standard value of 9.81 ms^-2

This means the velocity of the body changes by 9.81 ms^-1 EACH second.

It is a popular question to ask in physics exams what is the value of 'g' at the highest point of the motion of a body when it is thrown up.

The answer is still 'g' that is 9.81 ms^-2 and not "zero" unlike what many students are likely to answer. The final VELOCITY at that point however is zero.

2.   This problem can be thought as one belonging to a family a physicist would call "free-fall" because the only force acting on the parachutist is the earth's gravitational attraction.

You are given

    the initial velocity of the parachutist u = 0
    the distance travelled s = 80 m
    the time taken t = 4 s

What you don't know is his final velocity 'v'

The formula to calculate this is one of the equations of motion written as:

    s = average velocity x time
    s = 1/2[(u + v)]t
    80 = 1/2[(0 + v)](4)

Simplifying this you'll have

    80 = 1/2[v](4)
         = [v](2)
    40 = v

that is the velocity of the parachutist after 4 s is 40 ms^-1. The assumption you've made here is that the acceleration is CONSTANT and does not change throughout the motion.

Check:

There are other ways to solve this problem too and verify that you arrive at the same answer. The advantages of using equations of motion is you can use any equation that you think is convenient to use.

For example in this problem you could have used an equation to find the acceleration of the parachutist. To do that you can use the formula

s = ut + 1/2(a)(t)²

Here since u = 0, this equation becomes

s = 1/2 (a)(t)²

Making a the subject of the equation you'll have

    a = 2s/(t)²
    = 2(80)/(4)²
    = 160/16
    = 10 ms^-2

(just as you'd expect with free fall. The acceleration due to gravity is 10 ms^-2.

Using another equation to find out the velocity you'll find

    v = u + at
    = 0 + (10)4
    = 40 ms^-1

which is the answer as it should be. BOTH ways you've got the same answer. The only advantage with the first formula is you've got the answer in one step although it is not used so often.