Answer :

The answer is 2/5. Just divide each by two until in simplest form.
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Factors of 40: 1, 2, 4, 5, 8, 10, 20, 40
Factors of 100: 1, 2, 4, 5, 10, 20, 25, 50, 100
Common Factors: 1, 2, 4, 5, 10, 20
Greatest Common Factor: 20

[tex] \frac{40:20}{100:20}= \frac{2}{5} \\ \\ \frac{40}{100}= \frac{2}{5} [/tex]