Mrs Wong bought some flour.She used 3/8 of it to bake some cakes and kept the rest.What percentage of the flour did she keep?
If Mrs. Wong had a whole package of flour (1, or 8/8), and she used 3/8 of it, how much is left over?
Find that answer, and then convert it to a percent. I think that you should memorize the conversion between certain fractions and percents, like fractions with an 8 as the denominator. But if don't have them memorized at the moment, then set up a proportion.
For example, if I want to convert 1/8 to percent, I would write
$\displaystyle \frac{1}{8} = \frac{x}{100}$
and solve for x by cross-multiplying:
$\displaystyle 8x = 100$
$\displaystyle x = 12.5$
So 1/8 = 12.5%.
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