You are correct. 4 occupies the place in the octal system that 5 does in the decimal system; that is, it is half the base of the system; so .48 = (1)/(2) = .510.
and therefore 21.448 is closer to 228 than to 218.
Most decimal fractions do not produce terminating octal fractions. A safe rule to follow is to keep no more octal places than there are decimal places in the number that was converted, rounding off the octal fraction as appropriate.
Convert .2910 to an octal fraction, rounding the result to 2 octal places.
