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.
Given the chances of getting heads, or getting the answer correct is equivalent to 1/2 or 0.5, He has a 1/8 chance of answering all the questions correctly.
