First, let's note that after 12 minutes Marco is 2 miles from home and after 18 minutes he is 1 mile from home. This means that in 6 minutes (18 -12) we traveled 1 mile (2 - 1). Therefore, he travels 1 mile every six minutes.
Secondly, the problem tells us that after 12 minutes he was 2 miles from home. But now that we know that he rides 1 mile every six minutes we can say that on those previous 12 minutes he rode 2 miles.
Now we sum up those 2 miles he rode at first and the other 2 miles that he had left to ride (since by minute 12 he was 2 miles from home). Therefore the ride was 2 miles + 2 miles = 4 miles long.
