Question 715972
It's difficult to draw the Rate/Time chart but you can draw your own and fill in the information we gather here:

Bill
Rate: 4mph
Time: x (what we are looking for)

Larry
Rate: 12mph
Time: x-30min

Since we were not provided a distance, we will set them equal to each other using RATE*TIME

4x=12(x-30)
DISTRIBUTE THE 12
4x=12x-360
Get x on one side of the equation...substract 12x from both sides.
-8x=-360
DIVIDE BOTH SIDES BY -8 TO GET x ALONE
x=45 (positive 45 b/c a -/-=+)

Plug our solutions back in the chart.
Bill
Rate: 4mph
Time: 45
Larry
Rate: 12mph
Time: 45-30min=15min

We were asked to find the distance for Bill. 4mph in 45min. 45min=0.75hrs
4*.75=3miles

Always remember your units in your final solution, and always go back and double check exactly what the question asked you to find, don't just stop at "x="