Assume the CAPM holds. There are $3$ investors, A$,$ B$,$ and C$,$ and $3$ stocks, X$,$ Y$,$ and Z$.$ You observe that

A: holds $$6000$ of stock X and $$2000$ of stock Y;

B: holds $$1000$ of stock Y and $$2000$ of stock Z$.$

C: holds $$4000$ of stock X$.$

But you do not oberve A$\text{'}$s holding of Z$,$ B$\text{'}$s holding of X$,$ and C$\text{'}$s holding of Y and Z$.$

Based on your observations, you can conclude C holds $ of stock Z $($keep $2$ decimal places$).$