In joint variation y will varies jointly with x and w where :
y= kxw

For example :

A cake's cost varies jointly with the cake's thickness (T) and the square of it's radius (R). A cake 3 inches thick with a radius of 4 inches costs $7.20. What would be the cost of a cake that is 4 inches thick with a radius of 5 inches?

So we set up out proportion : 3 x 4^2 / 7.20 = 4 * 5^2 / x

We get : 48 / 7.20 = 100 / x

48x = 720

x = $15

We could also have said C = ktr^2
7.20 = k(3)(4^2)
7.20 = k(48)
k = .15

So then for out new problem : C = .15(4)(5^2) = $15

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The Cost C of printing a magazine is jointly proportional to the number of pages p in the magazine and the number of magazines printed m.
If the printing cost is $60,000 for 4000 copies of a 120 page magazine find the cost to print 2000 copies of a 9 page magazine.

We could set up a proportion 4000 x 120 / 60000 = 2000 x 9 / x

Solving for x we would get $2250.

We could also say : C = kpm or 60000 = k(4000)(120)

We find k to be .125 so in our second problem

C = (.125)(2000)(9) = $2250