When x is doubled, y is doubled. When z is multiplied by 5/8, so is y. The result of both changes is that y is multplied by 2·(5/8) = 5/4. The new value of y is
... y = 5/4·320 = 400
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If you really want to, you can use the formula
... y = kxz
and compute the value of k as
... k = y/(xz) = 320/(4·16) = 5
Then use this formula with the new values of x and z:
... y = 5·8·10 = 400
