The answer needs to be in mph, or "miles per hour", so let's convert "5 hours and 15 minutes" to pure hours (with no minutes attached)
Since there are 60 minutes in an hour, this means that 15 minutes = 15/60 = 1/4 of an hour (quarter of an hour). I'm going to convert the fraction 1/4 into decimal form 0.25; use a calculator or long division to compute this.
Therefore, 5 hours and 15 minutes = (5 hours) + (15 minutes) 5 hours and 15 minutes = (5 hours) + (0.25 hours) 5 hours and 15 minutes = (5+0.25) hours 5 hours and 15 minutes = 5.25 hours