SOLUTION: Suppose that y varies directly with x and inversely with z, and y = 18 when x = 15 and z = 5. Write the equation that models the relationship. Then find y when x = 21 and z = 7.

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Question 708694: Suppose that y varies directly with x and inversely with z, and y = 18 when x = 15 and z = 5. Write the equation that models the relationship. Then find y when x = 21 and z = 7.
Answer by KMST(5328) About Me  (Show Source):
You can put this solution on YOUR website!
y varies directly with x means y equals x multiplied times something else that does not depend on x.
y varies inversely with z means y equals 1%2Fz multiplied times something else that does not depend on z.
y=kx%2Fz with k being a constant (just a number)
says that y varies directly with x and inversely with z,
and does not depend on anything else.
When x=15 and z=5%7D%7D+%7B%7B%7By=18, so 18=k%2A15%2F5 --> 18=3k --> highlight%28k=6%29
So the equation that models the relationship is
highlight%28y=6x%2Fz%29
When x=21 and z=7
y=6%2A21%2F7 --> highlight%28y=18%29