step 4:

and what value has ?

a) a more real life example: If you have 10 candies and eat one, how many candies are left?Step 5 -> 10Iw1 – 10Iw2 = Iw1 +Iw2

Step 6 -> 10Iw1 – Iw1 = Iw2 + 10Iw2

Step 7 -> 9Iw1 = 11Iw2 ->question 2-> I get that 10 -1 = 9 and 10 +1 = 11, but why . is this allowed?

...

b) a more formal attempt:

That means that you have in this line

two summands with a common factor: