To round to the nearest tenth, you have to look at the number following the tenths digit. If the number is greater than or equal to 5, you'll round up to the next tenth. If it's less than 5, you'll "round down", which is to leave the rounding digit the way it is.
Therefore, 310.286 rounded to the nearest tenth would be
310.3, because 8 is greater than 5.
