Civil engineering question?

An existing pipeline 16000m long and 750mm diameter supplies 36000m3/d. Ultimately 108000m3/d will be needed, so a second pipe will be laid besides the first. However, the immediate need is for 46800m3/d, so for the present the second pipe will be constructed for part of the way only. Find the second pipe’s diameter and length, assuming the total fall is unchanged.