The atomic weight of silicon is 28.0855. Round this 4 significant figures.

When in a number need to round up to 4 significant figures then, look at the number present at the 5th position. If the number is equal or greater than 5, the preceding number is increased by one but if the number is less than 5, then just drop the number.

In the given number, number at the 5th place is 5, hence the preceding number, 8 is increased by one and the number becomes 28.1