x - speed of the current
40+x - speed with currentÂ
40-x - speed against current
t-time
(40+x)*t=100
(40-x)*t=60
      40t+xt=100
  +   40t-xt = 60
      80t   =160
         t   = 2
40t+xt=100, Â
40*2+2x=100
80+2x=100
2x=100-80
2x=20
x=10 mi/h
10 mi/h speed of the current.