ratio and proposition

Question

X,Y and Z are 3 quantities.X varies inversely with Y when z is constant.Y varies inversely with Z when X is constant.When Y=8,Z=7 and X=30.Find X if Y=16 and Z=21

Answer 1

Three quantities are varying based on one another but their product always remain constant i.e 

The overall change (global change) is always constant

$\therefore \ 8 \times \ 7 \times 30 = X  \ \times 16 \times 21 \\  $

                   $\color{Red}{X=5}$