How to round with negative precision?

Question

Mathematics

There is a task in which it is required to write the round() function. There is an indication in the problem: "Notice that the precision can be negative, which means that the part before the decimal dot will be rounded".
Can you explain how rounding with negative precision works?
There is an example in the task: "my_round(123, -1) must return 120". And if the accuracy is -2, then what should be returned?

Answer 1

my_round(123,-2) = 100.
Precision says that all digits after this index must be 0. And the previous one may increase by 1, depending on the rounding rules.