It's not RATE of SPEED. It's either rate or speed....one or the other.
Just set up the following proportion:, with D being distance.
Now solve this for D, the distance you're looking for.
Never use the term "rate of speed". There is "rate" and there is "speed". When applied to a rectilinear movement, they denote the same conception. But the combination "rate of speed" is NEVER USED and means NOTHING. It only reveals that you don't know terminology; so be accurate and watch your terminology . . .