There'll be 60 parts of lemon.
Because: if you have 20 and cut them by half, you will have 40 halves(1/2 of a lemon). But then, if you only take a half of these 40 halves, there will remain 20 halves(1/2 of a lemon) untouched and 20 halves will be cut in half again. So there will be those 20 halves(1/2 of a lemon) that remained and some new 40 halves(1/4 of a lemon), so there will be a total of 60 parts of lemon.
