What is the reason why an investigation is planned

It's planned so you know what you are doing during the experiment. If you don't plan something ahead of time then you forget what you need to do. Also you need to remember specific formulas and numbers.

First step is always balance the equation:
n2h4 + 7 h2o2 --> 2hno3 + 8h2o 

so assuming that n2h4 is in excess and only moles of h2o2 are determining the product content then 7 moles of h2o2 give 2 moles of hno3
so:
7: 2
x: 0.08
x=0.28 moles of h2o2