I'm adding these three atomic weights (40.1+12.0+48.0) together, and I got 100.1
How can I round my answer to three significant figures? thank you!
you by adding now have four significant figures. When adding, you keep all place values that were significant.
if it had been
34 + 12.2+6.0=52. Cant do the tenths, as tenths was not significant in the first numeral.
To add a little clarification to the answer already posted.
The correct answer to 3 s.f. is 100. and the period at the end is necessary and it not the end of the sentence. That sometimes leads to confusion and another way to do it to write 1.00E2 and that way a period is not necessary. If you leave it as 100 and don't put a period you only have 1 s.f.
To round your answer to three significant figures, you need to consider the first three most significant digits in the number. In this case, your sum is 100.1. The most significant digit is 1, followed by the digit 0, and then the digit 0. Since all three digits are significant, you don't need to make any changes to the first three digits.
However, the trailing decimal zero is not significant in this case. To remove the trailing zero, you can round your answer to a whole number. Therefore, your rounded answer to three significant figures would be 100.