Let X be the distance.
Since Speed = Distance/Time. From first we can conclude that:
Sa = X-180/T, Sb=180/T
=> Sa=(X-180/180)*Sb (cancelling T from the equation)
Similarly from second run if we can conclude:
=> Sa=2X-100/X+100 (cancelling T again)
Equating both Sa and simplifying the equation we get X = 240.
Bank is 240 meters wide.