So, if y varies jointly with x and z, that implies a direct proportionality.
The equation will look something like this:
y = xzk
"k" is the constant of proportionality.
Now, when y = 30, x =4, and z = 5:
30 = (4)(5)k
Multiplied:
30 = 20k
divided 30 by 20:
3/2 = k
So your final answer is:
The constant of proportionality in the equation y = xzk when y = 30, x = 4, and z = 5, is 3/2 or 1.5.
Hope that helps!