What fraction can be replaced with 1.2 when estimating?

Answer 1

The answer depends on the accuracy that is required. 1.2 metres instead of 1.21 metres will be OK for most purposes but if you are trying to fit a 1.21 metre object when the space available is 1.2 metres, you are in big trouble!

Answer 2

There are infinitely many possible answers and they depend on what operations are carried out using the number and what other numbers are involved. As a general rule, though, any number in the interval [11.5, 12.5) could be rounded to 12.