The distance from City A to City B is 180 miles. While driving from City A to City B, Bob drives at a constant speed of 40 miles per hour. Alice leaves City A 30 minutes after Bob. What is the minimum constant speed in miles per hour that Alice must exceed in order to arrive in City B before Bob?	Choices
A) 45  B) 48  C) 50  D) 52  E) 54
The time it takes Bob to drive to City B is 180 / 40= 
<gadget id="calculator">180 / 40</gadget>
<output>9/2 = around 4.5</output>
4.5 hours.   Alice needs to take less than 4 hours for the trip.   Alice needs to exceed a constant speed of 180 / 4= 
<gadget id="calculator">180 / 4</gadget>
<output>45</output>
45 miles per hour.   The answer is 45.
<result>A</result>
The distance from City A to City B is 180 miles. While driving from City A to City B, Bob drives at a constant speed of 40 miles per hour. Alice leaves City A 30 minutes after Bob. What is the minimum constant speed in miles per hour that Alice must exceed in order to arrive in City B before Bob?