Question 57169
SORRY   DIDN'T READ THE ENTIRE PROBLEM. THEY ARE ASKING FOR THE FASTEST WHICH WOULD BE 750=100 OR 850 KM/H.

DISTANCE = RATE TIMES TIME
4000=[X+(X+100)]2.5
4000=(2X+100)2.5
4000=5X+250
5X=4000-250
5X=3750
X=3750/5
X=750
PROOF
4000=(750+850)2.5
4000=1600*2.5
4000=4000