Question

round 399 to the nearest tenth

Answer 1

If by tenth you mean "9" in the tens place (tenth is actually a decimal part, so you're probably mistaken), then the answer is 400.

Answer 2

399 rounded is 400; you round down if the last digit (tenth column in your case) is under 5. But if it is 5 or above then you round up.

This obviously doesn't apply if you are calculating with significant figures.

Hope this helped :)