Hi Nikola, is this a problem you found on Khan Academy? If so where? There aren't that many exercises on feet and inches, and I would not expect this in the rounding section either.

So in this problem what you want to do is the subtraction first, because the parenthesis go first. You can make the numbers into decimals at this point but it's easier to just subtract by unit. 82' - 78' = 4', and 11" - 8"=3", so the parenthesis equal 4' and 8", also know as 4 feet and 8 inches (the notation #' = # feet, & #" = # inches, and # means number, usually an integer to clarify) now to make this number & 78' 8" into a decimal you take a look at the fractions. 8" is 8/12 of a foot, which can be simplified to 2/3, or in decimal form, .6 repeating. So 4' 8" is approximately 4.66666666etc. feet, and 78' 8" is 78.6666666etc. feet. Then you just have to divide them. I'd recommend using a calculator preferably scientific then enter each value as fractions if you want to be really accurate, as opposed to rounding up and just doing a bunch of 6s followed by a 7. E.G.: 4*3=12, 12+2=14, so 4' 8" is 14/3, and 78' 8" is 236/3. Also you could take those fractions, divide, and then calculate them into decimals.