Y = ax + b

         Y = a*x    + b
I)      20 = a*400  + b
II)    160 = a*1600 + b

I)
        20 = a*400  + b
        b  = 20 - a*400 

II)
       160 = a*1600 + (20 - a*400)
       160 = a*1600 +  20 - a*400
  160 - 20 = a*1600 - a*400
       140 = a(1600 - 400)
         a = 140 / 1200
         a = 7/60

I)
        20 =         a*400 + b
         b = 20 - 7/60*400
         b = 20 - 140/3
         b ~ -26.7

-------------------------------------
         Y = 7/60*x - 26.7   [km]
         x = 60/7*(Y + 26.7) [ms]
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