In order to calculate the rate (that is, the speed) David's dad was driving in miles per hour, first let's convert the time from minutes to hours using a rule of three:
[tex]\begin{gathered} 1\text{ hour}\to60\text{ minutes} \\ x\text{ hours}\to20\text{ minutes} \\ \\ 60x=20\cdot1 \\ x=\frac{20}{60}=\frac{1}{3} \end{gathered}[/tex]Now, to find the speed, we just need to divide the distance by the time:
[tex]\text{speed}=\frac{25}{\frac{1}{3}}=25\cdot3=75\text{ mph}[/tex]So the speed is 75 mph, therefore the answer is the third option.