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put this solution on YOUR website!d = r * t, so r = d / t
"Mike made a trip in 40 hours" __ r = d / 40
"Joe made the same trip in 30 hours because Joe was traveling 6km per hour faster" __ r + 6 = d / 30
substituting __ (d/40)+6 = d/30
multiplying by 120 (LCD) __ 3d + 720 = 4d
subtracting 3d __ 720 = d