The important factor here is the relative difference in speed between the two runners. Mary is running 2 miles an hour faster than Janet, so in effect from Janet's reference frame the gap grows as if she is standing still and Mary is running at 2 miles per hour. Therefore, to determine when the gap will grow to 1.5 miles we take 1.5 miles and divide it by 2 miles/hour to yield 0.75. Thus it will be 0.75 hours, or 45 minutes, before the two are 1.5 miles apart.