There are three methods we can use
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Method 1:
Reduce 8/24 to get 1/3 (divide both parts by the GCF 8)
Reduce 5/15 to get 1/3 (divide both parts by the GCF 5)
We get the same result of 1/3, so the two original fractions are proportional
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Method 2:
Set them equal to each other and cross multiply
8/24 = 5/15
8*15 = 24*5
120 = 120
Since the last equation is true, this means the first equation is true
Because the two fractions are equal, they are proportional
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Method 3:
Use a calculator to find that
8/24 = 0.333333... where the 3's go on forever
5/15 = 0.333333... where the 3's go on forever
We get the same decimal result, so the two fractions must be equal; therefore they are proportional