The express elevator will go 4 floors per seconds.
We have to find the rate of the express elevator before we can find how many floors one can go in one seconds.
time = 12 3 / 4 = 51 / 4 seconds
number of floor = 51 floors
rate = 51 / 51 / 4
rate = 51 × 4 / 51
rate = 4 floors per seconds
Therefore, the elevator will go 4 floors per seconds.
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