Jane has some pens in Box $$A$$ and Box $$B$$. Each of the boxes has $$10$$ blue pens, $$15$$ red pens, $$20$$ black pens and $$25$$ green pens. If $$20$$ pens are taken from Box $$A$$ and placed into Box $$B$$, how many pens must be returned from Box $$B$$ to Box $$A$$ to ensure that Jane has at least $$15$$ pens of each colour in Box $$A$$? 
Overseas Competition->Knowledge Point->Word Problem Modules->Questions Involving Sum, Difference and Multiples->Problems Involving Sum and Difference