Hi, thank you.
Would it be alrite if I multiply 3:5 by 2 instead?

Then the difference in units would be 5u for local n foreign stamps.

So 5u=15
Hence 6u=18

Hi, the key to this question is to make sure that the after units minus the before units for both local(10 units)and foreign(10 units) must be the same since both add the same amount of stamps(15 stamps). That is why I multiply 2 and 4 for before and after respectively to make sure that when I subtract, I get the same number of units.

Alright thank you:)