How do you turn two digit numbers in to tens?

Answer 1

If the second digit is 1, 2, 3 or 4 replace it by 0.

If the second digit is 6, 7, 8 or 9 replace it by 0 and increase the first digit by 1.

If the second digit is 5, then,

if the first digit is even, replace the second digit by 0,

if the first digit is odd, replace the second digit by 0 and increase the first digit by 1.

Many people will require that 5 as the second digit should be treated the same as 1-4. This ignores the upward bias introduced and IEEE 754 standard for rounding.