31.75 rounded to the nearest tenths

The answer is 31.8 because when you round something if the last number is 5 or above then you round the next number in front of it up by 1. And if the number is below 5 then you don't do anything to the number. The tenths decimal place is the first number after the decimal point and the second number after the decimal point is the hundredths decimal place. 

Ican’t answer if there is no diagram

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.