Calculating the average rate of ammonia production

[s3a]

Question:
http://img33.imageshack.us/img33/7647/questionb.jpg

I use molar ratio and get (8mol/L-0.8mol/L)/60min = 0.12mol/L/min but the answer is 0.14mol/L/min. What am I doing wrong?

Any help would be greatly appreciated!
Thanks in advance!

[s3a]

Actually I think I know how to do it:

(2(4mol/L/min-0.6mol/L/min))/50min = 0.136mol/L/min ≈ 0.14mol/L/min

Is this correct?