3/4+7/12=x. To be able to add fractions, you have to find the least common denominator for them, which turns out to be 12. To be able to get 4 in the first fraction to equal 12, you have to multiply that 4 by 3. Anything you do to the bottom, you must to do the top. So the ending equation would be 9/12+7/12=x. When adding fractions with the same denominator, you leave the denominator alone and only add the numerators. It would add up to 16/12, which is also 1 4/12, which simplifies to 1 1/3. Your ending answer would be 1 1/3 miles, or else C.
Hope that helps :)
