Hint: What fraction of the length of the bridge in on the banks?
We know that 1÷7 + 1÷8 of the total length is on the banks of the river, and 1÷7 + 1÷8 = 15÷56.
So the river width (4,100 inches) is the remaining 41÷56 of the bridge.
41÷56 x length = 4100
length = 4100 x 56÷41
length = 5600