I could not think of any other approach except trial & error, so I assumed that the total work units on the task are 600 and when completed in 12 hours, A & B together put up 50 work units per hour.
Now I made assumptions, since A works faster than B - I assumed rate of A & B as 30 and 20. So if A completed 1/2 of the work (i.e. 300 work units) first in 10 hours (300/30) and B completes the remaining work in 15 hours (300/20). But this would not work as total time was 25 hours. I needed to push it more.
Second iteration was rate of A as 40 and B as 10. So A completed his work in 7.5 hours (300/40) - mini eureka moment as I realized 7.5*4 will be 30 and B will take 30 hours matching the equation perfectly. Therefore with the rates of A as 40/hr and B as 10/hr - arrived at the conclusion that A works at a speed which is 300% greater than B.
