A man standing on the bank of a river 1 mile wide wishes to get to a point directly opposite him on the other bank. He can do this in two ways

a) head somewhat upstream, so that his resultant motion is straight across

b) Head toward the opposite bank and then walk up along the bank from the point downstream to which the current had carried him

if he can swim 2.5 miles/hr and can walk 4 miles/hr and if the current is 2 miles/hr which way is fastest and by how much?