If yy varies jointly with xx and zz, what is the constant of proportionality when y=30y=30, x=4x=4, and z=5z=5?

Given that y varies jointly with x and z, then
y=kxz
where
k is the constant of proportionality given by:
k=y/(xz)
since
y=30 when x=4 and z=5
then
k=30(4*5)
k=1.5
thus the formula will be:
y=1.5xz
Thus the constant of proportionality is k=1.5

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