Answer:
a) 
b) 
c) 






d) If we find the limit when t tend to infinity for the function we have this:

So then the number of words per minute have a limit and is 95 as t increases without bound.
Step-by-step explanation:
For this case we have the following expression for the average number of words per minutes typed adter t weeks:

Part a
For this case we just need to replace the value of t=10 in order to see what we got:

So the number of words per minute typed after 10 weeks are approximately 27.
Part b
For this case we just need to replace the value of t=20 in order to see what we got:

So the number of words per minute typed after 20 weeks are approximately 54.
Part c
For this case we want to solve the following equation:

And we can rewrite this expression like this:


Now we can divide both sides by 8.5 and we got:

Now we can apply natural log on both sides and we got:


And then if we solve for t we got:

And we can see this on the plot 1 attached.
Part d
If we find the limit when t tend to infinity for the function we have this:

So then the number of words per minute have a limit and is 95 as t increases without bound.