Answer:
b. n = 7
Step-by-step explanation:
You want to solve the proportion ...
5/15 = n/21
This is most easily done by multiplying by the inverse of the coefficient of the variable:
(21/1)(5/15) = (21/1)(n/21)
7 = n . . . . . . . . . simplify
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Additional comment
The math is a little easier if you simplify 5/15 to 1/3. Then, n = 21/3 = 7.
