when you set up equations like this, look at the units for help. They cancel out during division and multiplication like numbers to give desired quantities. Rates can be reversed if need be (3 meters/sec or 1/3 sec/meter).
time × (distance / time) = distance
See.. time/time = 1; leaving distance
Subtract the rate of the opposing walkway...
t × (3 m/s - 1 m/s) = d
2t = d
Answer is then A.
