The value of y varies jointly with the values of x and z. when x = 4 and z = 16, the value of y is 320. when x = 8 and z = 10, the value of y is?

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

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