Thread: shortest path

hi,
my question in the attachment.
I don't understand the shortest path model that he drew which I should complete

I just need one help me only in understanding the part of model he drew
for example what the nodes means , he said the age beginning of year
in year 2 when he buy new one why the age is 1 ?
how he calculate the cost on arcs? for example in arc between START AND 3
it's 23000 but based on what I understand it should be (42000*0.5)+5000 =26000

really I don't want the solution , I just need help in understand the question

Thank you all