18.the equation that describe y varies jointly as x and y = kxz then k is obtained by? a. k= xyz b. k= y/xz c. k= z/ky d. k= xy/z 19.if two variables x and y are so related that their product is constants then y is always equal to which equation? a. Y=kx b. y=xk c. y=k/x d. y=x 20.if a bariable z varies directly as a bariable x and inversely as another variable y then what kind of bariation it is? a. Linear b. Joint c. Inverse d. Combined​