Each piece of board is being cut into 5, so therefore for every one piece of board there will be 5 pieces.
So for pine you would do 100x5 and oak you would do 65x5
That would be your step 1 and 2 which eliminates C and D.
The question asks to find the difference between the two. Which means you need to subtract them to find the difference between the two numbers.
Option A would be correct.
Hope this helps! :)