Answer: a = 18
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Work Shown
I'll show two methods to solving this.
There may be other approaches.
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The first method would follow this set of steps.
3/a = 1/6
3*6 = a*1
18 = a
a = 18
In the 2nd step, I used the cross multiplication rule.
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The second method would follow these steps.
3/a = 1/6
3/a = (1/6)*(3/3)
3/a = 3/18
a = 18
I multiplied the 1/6 by 3/3 so that the numerator 1 turns into 3. Afterward, the two fractions have the same numerator. Therefore, the denominators 'a' and 18 must be the same.