Answer:
Joint variation states:
if y varies jointly with x and z
then the equation we get;
where, k is the constant of variation.
As per the statement:
If y varies jointly with x and z
Using above definition we have;
⇒
....[1]
y = 200 when x = 8 and z = 10
Substitute these value in [1] to solve for k;

⇒
Divide both sides by 80 we have;
2.5 = k
or
k = 2.5
then we get an equation:

We have to find x when y = 165 and z = 11.
then;

⇒
Divide both sides by 27.5 we have;
6 = x
or
x = 6
Therefore, the value of k = 2.5 and value of x is 6 when y = 165 and z = 11.