In a binary world, fractions where the denominator is a power of two (like 1/2, 1/4, 1/8, 1/16 etc.) are exact. All other fractions are approximate. We are used to 1/3 being approximated like 0.3333 (mostly). It takes getting used to the idea that 1/10, which works out so nicely with decimal numbers, is not at all nice with binary arithmetic.
In theory it is possible to know exactly how any number will be represented in binary arithmetic, if it will be larger or smaller than the "real" value, but the math is tedious. The way I answer questions like this is to use one of the converters that are available on the web. Or I just assume that however the computer approximates will be exactly the least helpful alternative (a variation on Murphy's Law).
Both Patrick's and my suggestions are standard ways of dealing with these problems. Use whichever is most helpful for your needs.