Respuesta :

 y varies directly with x means y =kx
y varies inversely with z means y =k/z .
 with  k  being a constant

 y varies directly with x and inversely with z
y=kx/z
x=15,z=5 so by putting values
18=15k/5
k=6


So the equation that models the relationship is
y=6x/z
y=6*21/7
y=18
hope this helps

The equation that models the relationship is as follows:

y = k x / z

The value of y when x = 21 and z = 7 is 18

Proportional relationship:

y varies directly with x and inversely with z. Therefore, this relationship can be represented as follows:

  • y ∝ x ∝ 1 / z

y = k x / z

where

k = constant of proportionality

y = 18

x = 15

z = 5

18 = 15k  / 5

cross multiply

90 = 15k

divide both sides by 15

k = 90 / 15

k = 6

Let's find y when x = 21 and z = 7

y = 6 × 21 / 7

y = 6 × 3

y = 18

learn more on inverse and direct relationship here: https://brainly.com/question/24824457

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