The sum to infinity of a geometric series with first term a and common ratio r is 10. The sum to infinity of a second geometric series with first term a and common ratio 2r is 15.
a] find the value of r. 
b] find the sum of the first four terms of the series, giving your answer correct to two decimal places. 
Part b I will be okay with, just the first part is the trouble here.
I got two simultaneous equations from it:
10 - 10r = a - (1)
15 - 30r = a - (2)
Putting them together, I got 5 + 20r = a
Or, 5 (1 + 4r) = a, whichever is preferred.
So this means (if I'm right) that = 5.
If this is right, where do I go from here?
If it is wrong, please could you help me?