Question

how to correct to 3 significant figures to these values
a)20.961
b)0.275
c)15.62 and 3.555

Answer 1

a) 21.0 Round the 6 up to get 21, then add a 0 after the decimal. 0's after the decimal count as a sig fig.
b)0.275 is correct, 0's in front of the decimal place just help to show you where it is and don't count toward sig figs.
c)15.6 Only round up numbers when they're 5 and up.
d)3.56 Numbers 5 and above round up.

Answer 2

Significant figures comes down to a short list of rules:
1) ALL non-zero numbers (1,2,3,4,5,6,7,8,9) are ALWAYS significant.
2) ALL zeroes between non-zero numbers are ALWAYS significant.
3) ALL zeroes which are SIMULTANEOUSLY to the right of the decimal point AND at the end of the number are ALWAYS significant.
4) ALL zeroes which are to the left of a written decimal point and are in a number >= 10 are ALWAYS significant.

In your case:
20.961 > round up to 20.1
0.275 = 3 sig figs, no need to fix.
15.62 > Round to 15.6
3.555 > Round to 3.56

Blatant copy and paste from this lovely source:
http://www.usca.edu/chemistry/genchem/sigfig.htm

Answer 3

@barnes 21.0 not 20.1.

Answer 4

Typo...I refuse to accept I literally put that in intentionally. (I most likely did... :( )

Answer 5

I'd give you another medal if I could, for effort.