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/T, Sb=X+100/T

=> Sa=2X-100/X+100 (cancelling T again)

Equating both Sa and simplifying the equation we get X = 240.

Bank is 240 meters wide.