Time taken by car = 25 hours
(4D - 500)/3X = 25
D = 75X/4 + 125
Speed of boat in still water = 'X' km/h
Speed of stream = 'Y' km/h
(D - 20)/(X - Y) = 30
75X/4 + 125 - 20 = 30X - 30Y
3X - 8Y = 28 ------(1)
(2D + 80)/(X + Y) = 45
2(75X/4 + 125) + 80 = 40X + 40Y
X + 6Y = 44 -----(2)
From eqn (1) and (2) -
X = 20 and Y = 4
D = 75*20/4 + 125 = 500 km
Z = (2D - 40)/XY = (2*500 - 40)/(20*4) = 12
I: 2X - 7Y < Z => 2*20 - 7*4 = 12 = Z
II: 3Y + 1 > Z => 3*4 + 1 = 13 > Z
III: X + Y = 2Z => 20 + 4 = 24 = 2*12 = 2Z
Hence, option 'd' is correct.
Study Notes
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