Flow will get an split based on equal pressure dorp in both the lines so in this case velocity has to be same in both lines by a factor of difference in length of the pipe. (assuming other fittings pressure drop as same).
So Smaller pipe is smaller in length also therefore, if velocity is V in large pipe then it can be 50/40*V = 1.25 V in smaller pipes.
Now since dia is 4 time lesse than area is reduced by 16 times so on eual level this velocity of 1.25V has to be reduced by a factor of 1/16 so it becomes = 0.0781 in terms of volume flow.
So in samller pipe it will be ~78 M3/hr if it is 1000 M3/hr in larger pipe.