If the last significant digit is followed by 5, 6, 7, 8, or 9, round that digit up. For example, 45.672 rounded to three signigicant figures is 45.7.

If the last significant digit is followed by 0, 1, 2, 3, or 4, round the number down. For example, 4212.8 rounded to two signigicant figures is 4200.

To round the number to the nth decimal place means to shorten the number so that the last significant digit is in the nth decimal place. For example, 42.12802578 rounded to the second decimal place is 42.13.