First let's find how many miles you travel per hour. To find that, you divide 45 by 3. This gives you 15 miles per hour. Now we need to find how many miles in half an hour. To do this we are going to convert into minutes. We need to find out how many miles per minute. There are 60 minutes in 1 hour, so we will divide 15 by 60. That gives 0.25, or 1/4 mile per minute. So now that we know how many miles per minute, we can find out how many for half an hour. Half an hour is 30 minutes, so we are going to multiply 1/4 by 30. You will get 15/2. We need to make that a mixed number, and that gives us 7 1/2 miles per 30 minutes, or 7 1/2 miles per half hour.
I hope this helped :)
(edit: I just realized I did this the hard way. Sorry. The easier way would be to just divide 15 by 2 since it would take two halves of 15 to make the amount of miles per hour.)
