For [tex]z[/tex] and [tex]x[/tex] to be inversely proportional, there must be some constant [tex]k[/tex] such that
[tex]zx=k[/tex]
Given that [tex]z=0.875[/tex] when [tex]x=16[/tex], it follows that
[tex]0.875\cdot16=k\implies k=14[/tex]
Then when [tex]x=21[/tex], we have
[tex]21z=14\implies z=\dfrac{14}{21}=\approx0.667[/tex]
