y varies jointly as x and z means y = kxz.
You can mix direct and inverse: y ∝ x/z means y = kx/z
Example: z varies directly as x and inversely as y. When x = 3, y = 2, z = 6.
z = kx/y
6 = k × 3/2 ⇒ k = 4
So z = 4x/y
When x = 5, y = 10, z = 4 × 5 / 10 = 2
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Solve:y varies jointly as x and z. When x = 2, z = 5, y = 30. Find y when …
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