SOLUTION: z varies jointly with x and y. When x=5 and y=2, z=70. Find z when x=4 and y =8.

Question 1191489: z varies jointly with x and y. When x=5 and y=2, z=70. Find z when x=4 and y =8.

Answer by Theo(13342) (Show Source): You can put this solution on YOUR website!
joint variation formula is z = k * x * y
when x = 5 and y = 2 and z = 70, formula becomes:
70 = k * 5 * 2
simplify to get:
70 = 10 * k
solve for k to get:
k = 7.
when x = 4 and y = 8 and k = 7, the formula becomes:
z = 7 * 4 * 8
solve for z to get z = 224
that's your solution.
k is the constant of variation.
it remains the same when the other variables in the equation change.
solve for k first, then use k to solve for another of the variables, such as z, in this case.

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