Yesterday's price of notebook = $3.45
Today's price of notebook = $3.20
We have to determine the percentage decrease in the cost of notebook.
Percentage decrease = (Decrease
Old price) 
So, percentage decrease = 
=
= 7.246%
= 7.25%
Therefore, there is 7.25% of decrease in the cost of the notebook.
Rice: .5(8) = 4kg (4000 g)
Popcorn: 150(12) = 1800 g
Total mass = mass of rice (4000 g) + mass of popcorn (1800g)
So...
Total mass = 5,800 g (or 5.8 kg)