Each women had 30 marbles.
First women : 3 marbles per paise, then she would have sold 10 sets of marbles(cheap marble)
Second women: 2 marbles per paise, then she would have sold 15 sets of marbles(costlier marble)
When the two types of marbles were combined, we got 60 marbles, 5 marbles per 2 paise. No of sets would be 12 sets.
As you can see, after selling 10 sets the cheapest marbles will be over. the remaining 2 sets i.e 10 marbles will be of costlier kind.
Till the 10 sets the combination is 3 cheap + 2 costlier
Now after selling 10 sets, 2 costlier + 2 coslier + 1 coslier makes a set
the cost of this combination should be 1 paise + 1 paise + 1/2 paise = 2 1/2 paise.
But the women sold this combination also for 2 paise, that is why we got 1 paise difference