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