Answer:
.009 miles/hour
Step-by-step explanation:

This is the method you should take with unit conversions in general, just set up fractions so the units cancel out, like the 7.5 feet cancels out with the feet in the 1/5280 so miles is left on top.
Answer:
967 times
Step-by-step explanation:
3 / 3499 is 1166.33333
3 / 6399 is 2133
Sorry, my answer was wrong. I can't delete it so as not confuse anybody i changed it to this.
I assume you are asking one of three things, so I will answer all of them.
if you are asking what is 1/2 of 312, then multiply 312 by 1/2. 312 is really just 312/1, so

we multiply fractions by multiplying straight across

if you were asking what part of 3 1/2 is 1/2 then we have to recognize what 3 1/2 is as an improper fraction. to do this, we convert 3 into halves. how many halves of pizza are there in 3 whole pizzas? 6. so we have 6 halves and one half, which is a total of 7 halves.

now, we do the same thing as the first problem

if you were asking what is half of 3.12, then we convert 1/2 to a decimal and multiply