if the following are true:
value of cell 3 at time 18 = (value of cell 3 at time 17) + (value of cell 4 at time 17)
value of cell 3 at time 17 = 2
value of cell 4 at time 17 = 3
then value of cell 3 at time 18 = 5

View as a table | Try proving it

Start from the conclusion and work back up the proof. Click the arrow to show the parents.