How do you round 64 to the nearest ten?

Answer 1

The tens' digit is 6.

If what follows it is less than 5 then replace every digit following the tens' digit by 0s. If what follows is bigger than 5 then add 1 to the tens' digit and replace every digit following the tens' digit by 0s. If it is exactly 5 then make sure that the tens' digit is even and replace everything after it by 0s.

Many naive teachers require that you round up (add 1 etc) when the next digit is 5. This introduces an upward bias and in statistically unsound. The solution is to round to odd or round to even. (See link). Mathematically, they are equivalent but, judging by the latter seems to be winning. It is the default mode in IEEE 754.