jmc

We use that fact that $d=rt$. Let d= distance, r= rate or speed, and t=time. In this case, let $x$ represent the time. On Monday, he was at a rate of $5\text{ m.p.h}$. So, $5x=2\text{ miles}⟹x=\frac{2}{5}\text{ hours}$. For Wednesday, he walked at a rate of $3\text{ m.p.h}$. Therefore, $3x=2\text{ miles}⟹x=\frac{2}{3}\text{ hours}$. On Friday, he walked at a rate of $4\text{ m.p.h}$. So, $4x=2\text{ miles}⟹x=\frac{2}{4}=\frac{1}{2}\text{hours}$. Adding up the hours yields $\frac{2}{5}\text{ hours}$ + $\frac{2}{3}\text{ hours}$ + $\frac{1}{2}\text{ hours}$ = $\frac{47}{30}\text{ hours}$. We now find the amount of time Grandfather would have taken if he walked at $4\text{ m.p.h}$ per day. Set up the equation, $4x=2\text{ miles}\times 3\text{ days}⟹x=\frac{3}{2}\text{ hours}$. To find the amount of time saved, subtract the two amounts: $\frac{47}{30}\text{ hours}$ - $\frac{3}{2}\text{ hours}$ = $\frac{1}{15}\text{ hours}$. To convert this to minutes, we multiply by $60$. Thus, the solution to this problem is $\frac{1}{15}\times 60=\boxed{4}$