first there is ones then to the right there is a decimal you are going to be adding the "<em>ths" (</em>EX. hundredths, tenths, thousandths) so you are going to have 8 hundredths (.08)
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The key signature allows the composer to use less accidentals which makes the music less cluttered. Just like the other person who answered this said when a score with multiple instruments is written it is almost always all in the same key to keep the music organized (their answer is better)
