Keep it simple. Figure out the difference between the wind and the runway heading, e.g. if you're on 31 and the wind is 330, it's 20 degrees. If it's 60 or less, divideit by 60, e.g. for 20 degrees it's 1/3. And multiply that by the wind strength, e.g. if it's 15 knots, you have a 5 knot crosswind component. This is easy to do in your head, and is at least as accurate as the information you're being given in the first place. If the difference is more than 60, take the wind strength as the crosswind.
Details... the error is never more than 14% (at 60°). The wind direction and strength are less accurate than that. You don't need an accurate number - what are you going to do with it? The only reason you need to know anything at all is to know whether you even want to try the landing. So if the wind strength is less than your crosswind limit (set by you, the aircraft, or your school), it's of no importance. If you're trying to land in a 40 knot wind it does matter, but that doesn't happen often and you'd better be prepared to go around at every 10th of a second.
Of course for a written exam they want the answer to the nearest 100th of a knot, and for that you do need some kind of chart or calculator. But don't confuse what happens in a written exam with actual flying.