Problem:
If z varies jointly as x and y, z = 60 when x = 3 and y = 4. Find y when z = 80 and x = 2.
z = kxy ; k = constant of variation
60 = k(3)(4)
60 = 12k
[tex]\[\begin{array}{l}\frac{{60}}{{12}} = \frac{{12k}}{{12}}\\\\k = 5\end{array}\][/tex]
z = kxy
[tex]\[\begin{array}{l}\frac{z}{{kx}} = \frac{{kxy}}{{kx}}\\\\y = \frac{z}{{kx}}\\\\y = \frac{{80}}{{5(2)}}\\\\y = \frac{{80}}{{10}}\\\\y = 8\end{array}\][/tex]
Answer:
y = 8
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