Zakat on Investment through others

1. I have invested money through somebody and there is no specific information about the property (there are many investors) share and current market value. How will I calculate Zakat? 2. I have purchased a plot on installments (file only as the balloting is not yet done) with partial payment. This is for future utilization of my kids. Do I need to pay Zakah on this and how to calculate the Zakah?