Finding common ratio and more?

I actually don't even know what topic this is. O.o"

It's a three part problem and I will probably ask lots of questions because I don't think I've learned this yet.

An infinite geometric series whose terms are positive has a finite sum. The ratio of the sum of the first four terms to the sum of the first two terms is 13:9.

a) Calculate the value of the common ratio r.

b) If the fourth term is 16/9, calculate the sum of the infinite series.

c) Find the least value of n for which the sum to n terms differs from the sum of the infinite series by less than 0.01.