When Mr. Peters drives from Boston to Worcester it takes him 30 minutes travelling at a speed of 60 miles per hour. Mrs. Peters drives from Boston to Worcester and leaves 5 minutes after Mr. Peters but travels at a speed of 90 miles per hour. Who will arrive first? By how many minutes?

Since the problem is requesting the answer in minutes, we are going to convert the speed of Mr. Peter to miles per minutes; to do that, we are going to multiply his speed by the multiplier [tex] \frac{60min}{1h} [/tex]:
[tex]60 \frac{miles}{h} * \frac{1h}{60min} =1 \frac{mile}{min} [/tex]

Now, to find the distance from Boston to Worcester, we are going to use the distance formula: [tex]d=vt[/tex]
where
[tex]d[/tex] is the distance
[tex]v[/tex] is the speed
[tex]t[/tex] is the time 

We know that the speed of Mr. Peter is [tex]1 \frac{mile}{min}[/tex] and his time is 30 minutes. Lets replace those values in our formula:
[tex]d=1 \frac{mile}{min} *30min[/tex]
[tex]d=30miles[/tex]

Now, lets concentrate on Mr. Peter clone, Mr. P:
Lets convert the speed of Mr. P to miles per minute:
[tex] \frac{90miles}{h} * \frac{1h}{60min} =1.5 \frac{miles}{min} [/tex]
We also know that they will cover the same distance, 30 miles. Lets replace the values in our formula one more time to find t:
[tex]d=vt[/tex]
[tex]30miles= 1.5\frac{miles}{min} *t[/tex]
[tex]t= \frac{30miles}{1.5\frac{miles}{min}} [/tex]
[tex]t=20min[/tex]
But since Mr. P leaves 5 minutes after Mr. Peters, we need to add those 5 minutes to M. P's time:
[tex]t=20min+5min[/tex]
[tex]t=25min[/tex]

We can conclude that Mr. P will arrive first, 5 minutes before Mr. Peter.