Problem Solving

I have a few problems that I encountered today that I don't seem to be able to solve. Please help me out.

#1
In the arithmetic sequence t1, t2, t3, ..., tn,..., t1 = 23 and tn = tn-1 - 3 for every n>1. What is the value of n, when tn = -4?

#2
The perimeter of square region S and rectangular R are equal. If the sides of R are in the ratio 2:3 what is the ratio of the area of Region R to the Region S?

#3
Two Water pumps working simultaneously at their respective rates, took exactly 4 hours to fill a certain swimming pool. If the constant rate of one pump was 1.5 times the constant rate of the other, how many hours would it have taken the faster pump to fill the pool if it had worked alone at its constant rate?

#4
In a stack of boards at a lumber yard, the 20th board counting from the top of the stack is immediately below the 16th board counting from the bottom of the stack. How many boards are in the stack?
I always get 36 but it should be 34?

#5
For which of the following functions F(a+b) = f(a) + f(b)?
Answer: F(x) = -3x????

#6
Adam and Beth each drove from Smallville to Crown City by different routes. Adam drove an average of 40 miles per hour and completed the trip in 30 minutes. Beth' route was miles longer and it took her 20 minutes more than Adam to complete her trip. How many miles per hour did Beth drive on average?

Thanks everyone!